This achievement can be obtained during CH.9 - THE PART WHERE HE KILLS YOU
In the beginning of the chapter, after the title card, you will be able to escape using some puddles of white gel.

After you evade the death trap, before proceeding too far, Wheatley will beg you to come back.
Return to the area near the death trap.
Wheatley will start convincing you to simply jump off the edge, into the pit.

Follow his directions, jump into the pit, and you will unlock the achievement!
Happy achievement hunting, and I hope this guide helped!