This is an INCREDIBLE
leg exercise. Let me just say that up front. If you're looking
for a way to truly trash your legs, this one will push you
right to the edge.
It allows you to use extremely
heavy weight but in such a manner that you can hit failure
at the BOTTOM and be done with the set by just falling down,
rather than with a standard squat where you have to get
the bar up to the top and re-rack it. Not necessary here.
You can push till you fall down, which to my mind, makes
it a great leg exercise :)
I call this one the Donkey
Barbell Squat because of your body position while you're
doing it.
You'll be setting this one
up in the power rack - set one safety rail at stomach level,
the other just above mid-thigh level when standing. You'll
have to adjust the settings for your own body dimensions
but this is where I found it.
I also recommend using a barbell
pad and sliding it down to the bottom of the lower side.
You're going to be resting the bar on your hips/lower back
area and pushing your legs to a straight position.
Here's what it looks like.

As you can see, I've got plates
on both ends of the bar (to keep the top end from flying
up when the bottom end is loaded). The top end here has
5 plates on it. The bottom working end has 7 plates on it.
This makes for a very heavy exercise!
Get underneath the bar, the
length of it running down your back, the bar pad on your
hips. Grip the higher end of the bar with both hands and
hold on tight! You'll be keeping your hands on the bar the
whole time.
Now just straighten your legs!

Sounds simple, yes, but when
you really load the bar up (as you can with this exercise),
it is TOUGH. You'll really able to push some big weight
with this one, though.
This exercise is like a cross
between a machine and a free weight exercise. You have some
stability like a machine and you're able to stop the exercise
at the bottom, like a machine. But you also are not locked
into the movement like a machine - the free movement makes
it more like a free weight version of a squat and leg press
combined into one.
And, as I mentiioned above,
you can REALLY push hard. Here's a picture of me after a
set of this exercise at this weight:

Definitely watch the video
- it'll give you the best idea of how this exercise is performed.
Then try this at the gym (don't start this heavy, of course,
and make sure your bar is rated to be able to handle this
weight if you DO go this heavy eventually - otherwise you
WILL bend the bar).
Be ready to have a lot of
people look at you funny...then ask where you learned it...then
watch THEM as they try it themselves the next day!