IPF: "7. Allowing the bar to return to the platform without maintaining control with both hands, i.e.: releasing the bar from the palms of the hand."
In other words, you cannot drop the bar. You have to maintain contact, or grip, on the bar until it's on the platform. There is no further detail beyond that so if the bar bounces up and you lose your grip, that is not a cause for a red light. Dropping the bar from lockout is, losing grip on the way down is. Some refs get a bit narky if you lower very quickly (i.e. drop the bar but stay with it) but the rules do not specify how slow or controlled the lowering may be.
Capo etc: "7. Allowing the bar to return to the platform without maintaining control with both hands." So, the same.
Themes the rules. Its all about control of the weight. If you start letting people throw chit around without control, you introduce a danger factor beyond a controlled lift. The idea is to use power to powerlift, not momentum or other tricksy Gollumesque funny business.