From Hard Drop - Tetris Wiki

The Grim Grotto is a mid-game prophecy trimming setup that yields a T-Spin Triple. Note that the order of the hang or trim are interchangeable.

[edit] Grim Grotto

The easiest way to remember is to look for the square shape. Make sure you use the right piece though. If you use the wrong S or Z, it will create an "empty" S or Z-Spin Triple instead.

Beware using the wrong piece!

[edit] Grim Triple Double

This is a greedier Grim Grotto (credits to XaeL):