For what it's worth, I think that if you're trying to strictly access concentration states, you shouldn't focus on "split seconds". In insight practice, you try to refresh your awareness often, not letting attention rest anywhere, always keeping it on new sensations as they come. But in strict concentration practice, you act as though your object of meditation, usually the breath, is one, continuous, stable thing. The breath is actually a myriad of sensations, not just a singular object. In vipassana, you want to break the breath down and see as many individual motions, sensations, and movements, as quickly as possible, at the quickest rate possible. But if using the breath as a concentration object, don't look at the motion, don't look at the change in breath, don't try to compare one moment of breath to another moment of breath.
Think of a river. In vipassana, your goal is to watch the motion, to see that it is never the same river from moment to moment, there are a million drops of water in constant motion and never remaining in the same place or configuration for even a second. It doesn't matter if the water is turbulent, calm, fast or slow (making an analogy, the river as breath). Contrast this to strict concentration practice, where all you want to see is the whole of the river. You tune into the calm flow. To be fair, the vipassana perspective (ever changing from moment to moment, not a breath but a constant flux of individual, changing sensations) is the truest nature of the breath, which means that even if you're trying to practice strict concentration, it's very easy to tune into the motion and flux/change of the sensations of breath.
But to access pure concentration states, you have to let go of the notion of investigating or scrutinizing or analyzing. No analysis in concentration. Your goal is to simply place yourself in the breath and stay there.
You also have to be very calm and resigned to do concentration. Don't expect, don't wonder where it should go, don't try to make the states arise, as they arise of their own accord when the conditions are correct. Your job is just to set up those conditions. Pretend your breath is getting a massage, and your focus on the breath is the massage. Let your awareness be comfortable and calm, with enough effort to stay focused, but not so much that it feels tight or strained or unpleasant. It's about getting into a really comfortable mindset. Think of the feeling of sinking into a nice bath (this is from MCT

, or sinking into a relaxing sofa after a hard day's work. Get yourself into the mindset of "ahhhh.... yeahhh.... this is nice".
Still, I think I haven't fully understood the moment-by-moment Samatha technology. I suspect what solidifying the sensation of breath means, but I cannot link the general concept with my actual practice. What should I focus on? I think I've understood what Daniel Ingram said about feeling the pleasure of breathing (like doing it within a flower garden), but I simply don't know what to do with my focus within passing seconds. Of course, I can follow my breath moment by moment: how it starts, rises, peaks, descends, stops and so on. But this technique - as I understood - is part of Vipassana practice, it's not Samatha. If I should just "go with it", then I don't know exactly with what should I go?
Again, don't focus on moment to moment if you're doing concentration practice. From moment to moment, you have an ever-changing set of sensations. Try to put that aside. Personally, I only focused on the start, rise, peak, descent, and stopping of breath in order to train myself to focus on the breath and not tune out to la-la land and lose focus. But if you're able to follow it on that level, maybe try to just see that there is breath in some form or another. Don't focus on one spot, just feel that you are breathing. You can be aware of it without stopping to say, "okay, now it's rising, now it's falling". Try to just see it as breath, no matter what phase the breath is at. It's still breathing when it's going in and when it's going out.
On a side note, I tried practicing concentration states before trying insight practice. I don't think I really ever hit any jhanas, and most of the time, I was just floundering. But after practicing vipassana, I was able to access jhanas. I suspect that it's due to the fact that in vipassana, you have to accept the frustration, the confusion, the uncertainty, the distractions, all as valid things to watch. If you can keep that perspective in mind when doing pure concentration practice, you can "let go" of distracting thoughts and sensations with less resistance.
Good luck! This is just what worked for me, it seems that the methods for entering concentration states can vary just as much as the way the concentration states are experienced. All that matters is finding out what works for you, even if it isn't perfectly in line with someone's particular method. Keep experimenting!
For me, it can look like this:
-Just starting, distracted, thoughts pull my attention away, various unpleasant feelings in the body (soreness, itches, impulse to adjust posture, etc)
-Focusing on the breath, disregarding distractions, not being upset when distractions happen
-Able to stay with breathing and not get distracted
-Perceiving the breath as pleasant, enjoying it, the enjoying of it makes it easier to stay focused on it (breathing can be neutral, but convince/fake yourself of the pleasantness and it will be real. Just like how you can make yourself smile artificially but it still gives you the feeling of happiness)
-Focusing on the pleasantness of the breath, steeping in it, immersing myself in it
-Pleasantness of the breath spreads through the whole body, like bathing in it. Sometimes it happens on its own and other times you kind of "extend" the pleasantness to the boundaries of the breath, extending it through the body.
-The whole body feels extremely pleasant, almost like some sort of high. (1st jhana)
-The pleasantness makes me not only physically happy, but mentally as well! Tuning into the mental aspect, the happiness/joy/mental positivity
-The happiness/joy begins to grow, and can really explode! Steeping in this joy, letting it expand till you're filled with immense joy (2nd jhana)
-After awhile, the intense joy wears off, and I'm left with a calm, pleasant, simple and quiet happiness. I let the intensity naturally drop away, as it's actually hard to sustain that intensity for long. It's usually the easiest transition.
-Things are quiet, the mind feels satisfied and the body feels so comfortable, there is a simple, silent and extremely calming happiness pervading the whole field (3rd jhana)
-After I've had enough, I see that even the simple happiness can be "dropped". Everything can be dropped, really. You enjoy it so you naturally stay with it, but you can indeed drop the happiness, let it go, let it pass. If concentration is high enough, then I enter into...
-A state where even emotional happiness drops away, and all there is is silent, wide, and all encompassing peace. (4th jhana) Don't mistake the happiness of 3rd jhana for this peace, because it's absent in the 4th. It's wide open, even more silent than 3rd, panoramic, spacious, completely unconcerned, completely un-involved, and extremely aware. Here, awareness touches things but doesn't get involved in them at all, like watching a basketball game that you have no stake in, whereas before you had preference for a team, preference for players on the court, etc.
I am not really qualified or experienced enough in formless realms to go on, but a strong 4th jhana is the base for continuing to higher jhanas.
I hope this wasn't too much info! Let me know if any of that needs clarification.
Edit: Most importantly, HAVE FUN!

It should feel fun and comfortable and pleasant, never unpleasant or confusing or doubtful! Treat it like such!