Essentially, it is like a laying down lat pulldown. At the top of the eccentric motion, you look like you are being crucified; hence, Crucifix Lat Pulldowns.
This is the set up: take the leg holders, and set them at around middle position. Once you have that setup, grip the bar and pull the cable out, and rest your pelvis on the leg holders, and your feet straight back behind you. So, at the start of the lift, your hands are in the air, gripping the bar at the widest position, and your body is lying completely flat, with pelvis on the leg holders, and legs completely straight. (Picture a horizontal crucifixion.) On the concentric, I alternate between pulling behind the neck, and in front of the face, as it concentrates on different muscle groups.
I usually pair these with rope pulldowns, on a 3 set super-set: 15 reps each set, 4 second eccentric, 2 second concentric, 2 second pause.