Hello Newt,
Welcome to the DhO and your first post.
Newt :
So I chose the breath as my object, sat down and committed myself to discovery with the firm resolution of not getting trapped in any feelings. After a few minutes of setting up mindfulness, I started to focus on the breath and soon discovered a very pleasant sensation similar to butterflies in the stomach. With every breath it became more pleasant and its scope inched outwards from my stomach.
After a while (10 to 20 minutes) I felt a strong tingling centered in the middle of my forehead that felt connected to the butterflies-in-the-stomach feeling. Once this hit, any distractions in my mind collapsed and it became really easy to focus. But still I kept being aware of the breath and these pleasant feelings.
You say this was a "strong tingling centered in the middle [of the] forehead." Are you certain it was a tingling sensation and not a sense of pressure, similar to a headache? I say this because the sensation of pressure (I like to describe it as a balloon expanding in the center of the forehead) is a strong indicator of the establishment of concentration or
samadhi.
Anything that actually
does feel like a tingling sensation can be an indicator of the kind of sensation that one can use to concentrate upon to draw one deeper into the absorption of the first
dhyana.
Newt:
Sometime afterwards, maybe 20 or 30 minutes, at the end of an out breath came this intense feeling of strobing warmth pulsating out from my centre and it felt like my whole body was subtly but strongly vibrating. I thought 'holy shit!' and lost my focus. I have the impression that this feeling lasted 6 seconds, but I wasn't counting.
People new to meditation will experience any number of strange or unfamiliar sensations while getting some experience in meditation. This sounds like one of those episodes. Nothing special here to hang onto. Move on.
Newt:
Sadly I haven't been able to reproduce the state. I can get close but everytime that eruption of vibration gets close, I think "This is it!" and I lose my concentration.
Don't be concerned about not being able to reproduce it. If it is meant to arise again, it will. It if does, do not cling to it. Just let it go. It is ultimately of little importance.
What I mean by that is the sensation is relatively unimportant in the bigger picture of things. In order to maintain such an "eruption of vibration" you have to practice equanimity, having neither a liking nor disliking for the sensation. Even mindedness, in other words. That takes extraordinary mental control, which is an ability you should be focusing on developing, as it is useful in the practice of heedfulness and mindfulness, both of which are key to the attainment of awakening. For the time being, use such experiences to develop equanimity toward the object of meditation.
In peace,
Ian