www.ilga.gov has a searchable and certified archive of the Illinois Compiled Statutes.
The Federal regulations regarding autos are certainly federal, but the Illinois state laws written in the Illinois Compiled Statutes are state law, I assure you.
If you can open the blade without touching the blade, by using a switch, stud, button, etc "in the handle" of the knife, then it's a "switchblade" under Illinois law and verboten. You may not carry, buy, sell, give, or possess such a nasty device.
By my amateur lawyer reading, this is the reason assisted-opening knives like the Ken Onion designs are legal in Illinois--you must push the blade part of the way open, even if you're only pushing on the "flipper"--and that "flipper" is still part of the blade, not a control in the handle. IANAL, but this conclusion was also reached by the Tazewell County State's Attorney when some Peoria-area police arrested a man for possessing a Kershaw designed by Onion, so I think it's pretty firm.
Concealment is irrelevant in Illinois law as regards knives. The bottom line is that any number of knives are illegal for carry
with intent unlawfully to harm another, and if they aren't specifically banned as "dirks, daggers, etc." then there's a clause which prohibits "dangerous knives."
This term is not defined anywhere in the Compiled Statutes.
Therefore, any knife any police officer finds on you is grounds for arrest--he can always claim that it is a "dangerous knife" and since such is undefined, the arrest at least will probably stand. In court, who knows?
However, again by my amateur wannabe lawyering, even a "dangerous knife" is only prohibited if you are carrying it with intent to hurt someone "unlawfully." So carrying it with no intent is perfectly legal. Carrying it for self-defense is perfectly legal. Carrying it to shank your meth connection and jack his supply of Pseudofed is illegal and can be tacked on when you're arrested for the other crimes you're committing, which seems to be the only real purpose of such a law.
There is no mention of concealing a knife or carrying one openly in the law--Illinois does not address this issue.
www.ilga.gov