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Parapet is a T-Spin Prophecy technique that is usually faster compared to the alternative, which is waiting for soft drop and tucking the L in. This alternative is not even possible if you rely on sonic drop instead of soft drop, so the Parapet option is a great tool to keep in mind to avoid bailing on an other-wise easy T-Spin double. Introduced by Chopin.

If the hole is deep enough, then this directly connects to a btb tetris.
"You do it if it doesn't ruin your downstack/APM capability. To be more general, stacking your 2 wide takes longer; otherwise you do it because its faster to finish off your opponent." -Paradox
If you have a 2-wide that you would ruin by placing it there, then you shouldn't." -Paradox
"It reduces the chances of a combo after the T-Spin double." -jkwon23