From Hard Drop Tetris Wiki
Powerstacking, a term coined by player Microblizz, is a stacking method where you overstack your well on the sides with a TSD slot and usually a Tetris column in the middle. The overstacked sides essentially becomes a combo setup. This stacking is easier to execute if you have the speed advantage against the opponent.
Powerstacking is a popular way to stack in games with fast piece movements because the sides do not have to be deliberately stacked, unlike other methods such as mid-game 4-wide. Combos that you send after the TSD and Tetris send messy garbage which is an added bonus.