It's massive misinformation they use to cause panic.
I have many liberal friends & other than "Knowing one exists" they have never ever been able to tell me exactly what they think it actually is in detail.
The synopsis seems to be you can buy anything at a gun show because its "illegal everywhere else" with no kind of anything other than a *nudge, nudge, wink, wink*, secret handshake or something they are kept in the dark about because only gun owners actually know the secret procedure & passwords.
They insist & get quite vocal about it "Knowing it can be & is done all the time with no fear or the law". When I challenge them to actually show up at a gunshow of their choice & prove it to me they never do. I guess it would cause disbelief if they saw the dozens of NICS checks being called in at gun shows because they simply don't understand (or want to believe) the real process.
The closest I've ever heard of in reality is that private transactions of non licencable items can be conducted without any additional rules than the exactly same transaction anywhere else. Basically if its legal in the area its legal at a gun-show as well, if it isn't then it doesn't suddenly become legal just because its at a gun show.
Every show I've ever been to has signs stating clearly that all transactions must comply with all federal, state & local laws, but to the believers that's just part of a smokescreen to hide the "real deal"!