I'm considering picking up a used/old clip on aero bar setup.
The one thing I'm confused about though, is what part of my arm is supposed to rest on the foam pieces.

Are my elbows supposed to rest on the foam pieces? Or maybe the rear part of the forearm?

Thanks.

Look at photos and figure it out.

I'm guessing we're talking road bike and shorty bars? Look at pro triathletes like Brownlee

Depends on the bars. Some are shorter, some a longer.

try before you buy. Mine made my front end wiggle.

tmi

Many are adjustable in length so you can dial in your position better and have your weight resting just before your elbow.

Even that might not be enough.

I had a set that worked fine at the LBS. Got 'em home, put 'em on my bike, and any attempt to transition to or from them was a scary death wobble. On the bars was great. Off of the bars was great. Something about the transition was just unstable though.

Spend some money and get a professional fit. Then the last thing on your mind will be trying out an aero bar set up.

My cheap Profile bars are nice and long and Fredly. My arms rest very close to be elbow. I have seen others that I don't actually know how they are ridable.