The day 125 meditation is both simple and extremely powerful. The basic idea is that placing our attention on something actually magnifies that thing. Thus, by paying extra attention to things that are pleasurable, we become attached, and ultimately addicted. Conversely (though Gates doesn't discuss this point) paying too much attention to things that are painful leads to aversion. That's pretty simple, and very clearly put.
Gates shows how this principle explains the power of asana practice. Asanas force us to focus our attention on the working of our bodies, on maintaining control of our breathing, and on learning stillness and peace in poses that are uncomfortable. The attention that we bring, day after day, to these positive things reaps benefits that seem to be out of proportion to the exercise itself.
It also helps to explain why asana practice seems to be so addictive. The focus that gets put into the asana practice does just what Gates says attention is likely to do: it leads to an attachment to the practice. But this is an attachment/addiction that I can live with.
And finally, the principle leads to a general prescription that is also very powerful If you have a bad habit, or something that is nagging at you, then substitute something positive for it and focus your energy and attention on the positive thing. By directing attention in a positive direction, we have the power to create good attachments. (I used to apply this principle to some of my cultural tastes, by refusing to listen to bad music, to watch bad movies, or to watch basically any TV at all. I've grown lax in that area, and I'm thinking maybe I should start paying more attention to stuff that really is good. )