lying leg curls.
Grab a velcro strap. Set the cable attachment to the bottom.
While seated, secure the strap around your ankles, binding them together (slightly apart).
Roll onto your stomach and shimmy your way out till your about a foot out from the machine.
Now, slow and steady, "curl" your legs in and out towards your glutes!
Be sure to engage and squeeze!
Repeat for however many reps you'd like!