This one is a variation
on the classic "leg raise" exercise...the
key difference here lies on WHERE you're doing the
exercise.
It's not a hanging version
- it's actually a fair bit harder than that!
With this version, you're
going to be supporting yourself on the safety rails
of the power rack - it almost looks a bit like the
Iron Cross position a gymnast gets into on the rings.
So first, set the safety
rails on the rack to about the level of the bottom
of your rib cage - you can adjust height any time.
I like to set the height
so that I can set my hands on the rails while in a
standing position, then bend my knees to get my feet
off the ground. I find this to be easier than trying
to jump up in order to have straight legs at the bottom.
The arms should be about
45 degrees at the shoulder.
Set your hands on the
rails and hold on! Bend your knees and get your feet
off the ground. You will instantly feel big-time tension
in the abs and torso because of how you're supporting
your body in this position.

Now we add in the leg
raise...

You can also do it as
a knee raise...

I like to start with
the straight(ish) leg raise then as I get tired, switch
to the knee raise in order to keep going.
This one is BRUTAL because
not only are raising the legs (which involves the
abs in a movement capacity), you're also involving
the abs very strongly in a supporting capacity.
Give this one a try
in your next ab-training session (do it first, before
any other ab exercises - trust me on this - you probably
won't need to do any more abs after a few sets of
this).