In Tetris DX, your save file has a statistic called "Power", which goes up when you do well, and goes down when you don't.
I never got the instruction book with my copy (bought second-hand), so I don't know if it's explained in there, but I'm not sure exactly how Power operates.
I don't know if it's level-based, score-based, lines-based, how well you play (e.g. number of Tetris' you get in a round or how long you spend fast-dropping blocks) or if it's something else, as every round is different.
As shown in the photo below I'm at Power level 1288. I can't seem to get it much higher than that, even though I'm regularly hitting 300+ lines (and at one point hit 975 lines).
So, what factors determine your power, and what's the highest it can go?