Every portcullis I have ever read about that was in working order was counter balanced so a single man could raise or lower it. (Note, this has not been a lot of material. It is an oddly lacking detail from a lot of books on fortifications.) This means that you can't really 'crash the gate shut' in general design, because there is a nearly equal weight that you have to raise up to lower the gate.

Now yes, you could possibly cut the cables holding it, but we're not talking about a thin little rope that you can easily slice through in the blink of an eye. Also I know a number of gates were eventually upgraded to metal chain in place of fiber rope, because chain doesn't easily rot out and let the gate fall at a random and potentially highly inconvenient time.

I also agree with rdanhenry about actually letting the gate crash down suddenly being a bad idea, as you risk damage to the thing that is suppose to be in good enough shape to withstand rams and the like. No sense doing part of the attacker's job for them.

And on another random note, I can't really think of many times where you would need to suddenly slam heavy gates shut. Often times if there was any worry of attack or possible attack, the gates would simply remain close. Either smaller secondary gates would be used, or possibly wicket gates set within the main gates themselves. Gates that weren't locked down firmly would have been guarded by a number of armed men, and the space from a large distance around the gate is cleared. If a large party that posed a threat to the castle appeared, the gates would have been barred before the enemies could get there. Now it is true that a small party could attempt to take a gate, but the defenders still have an advantage: A narrow passage with a few men armed with spears isn't a fun thing to walk through. Add in a few men above the gate throwing stuff down, and it becomes down right nasty even with the doors wide open.

Traps and surprises could be useful at some times, but you also have to consider the risks to the people who are actually suppose to be there. Hair trigger death gates would be kind of frowned upon if they killed someone they weren't suppose to.

Can anyone find a reference as to what castles a Herse was ever used on? It isn't something I've come across before, and now I'm really curious about it.