This exercise is not quite
a Zercher squat (where you hold the barbell cradled in the
crooks of your elbows, down by your stomach) and not quite
a front squat.
It's basically a combination
of BOTH, where you're supporting in the crooks of your elbows
but at shoulder height like a front squat.
This hits the abdominal wall
HARD - very similar to a curl squat but without the tension
on the biceps and without the possibility of resting your
arms on your abs as you come up.
Holding your arms at shoulder
level really puts tension on the core. It's a great way
to build up the core to improve it for front squatting.
Here's the cross-arm position
of a front squat - the bar is supported on your shoulder
girdle.

Here's the "Zercherish"
position...get the bar right in the bend of your elbows.

Now clasp your hands over
the top of the bar to lock them in.

The bar is now not being supported
on the shoulder girdle but being held up solely with muscle
support from the shoulders and frontal core muscles.
Set the bar up at regular
squat height - you won't need much weight for this one.
Hook your elbows under.

Step out of the rack.

Then squat!

Not a hard concept in general
- it's the bar placement that makes this one different.
Give it a shot - I promise your core will be screaming at
the end of the set!