What exactly is "Piston Slap?"
 

I've heard that it's literally the pistons rocking in their bores, beating against the cylinder wall. Is this correct? thanks :)

I believe that happens with aftermarket forged pistons that may run a larger piston to wall clearance than stock. They don't expand as quickly as stock pistons, so they tend to be noisy on startup.

yea, most of the piston slap ull hear is on startup....

pistons slap is anytime the pistons do not properly fit the bores and cause extreme sidewall contact.

the forged pistons situation is commonly referred to as "piston rattle" since the tolerances are spot on, but forged aluminum doesnt expand as quickly as cast aluminum.