I think you could say that the energy and effort you're expending is on investigating what you aren't able to let go of. If you're fighting with an emotion, you're going to need energy to watch yourself fighting without getting involved in it so you can see it as it is - or maybe, so you can see the pure awareness behind it. Once you have found rigpa/buddha nature/ground of awareness, though, you no longer need to expend energy because, in that moment, you're enlightened (this is how the teachings generally work). This coincides with Kenneth Folk's third gear practice, Tibetian Dzogchen, (and I would say actual freedom, but that's a whole can of worms).
I think noting practice could be the same or different depending on how you apply it. I don't really follow the MCTB methods, so don't take my advice on this too seriously, but I think rapid noting tends to bring you towards the "blip" style fruition where there is a break in awareness, whereas letting go supported by an occasional noting will lead you towards the living experience of "not-self"/effortlessness style fruition (I can vouch for this second one, but I can't compare the two). It's hard for me to say, after lots of reading, if these paths are mutually exclusive or tied together somehow, but I'm guessing the relationship is probably messy. It's impossible to know what the Buddha was pointing to, end of the day.
I would say give lots of stuff a try and stick with what moves you towards your goal.

Maybe someone who's done both methods could chime in. I've been stalking Nikolai around the web because he did the MCTB path, then moved on to actual freedom. He has a lot of good stuff about it on his
Hamilton Project blog.