Overhead chambers both thrust and overhead attacks. Perfect attack against polearm+shield users.
The best way to feel the timing is to hop on the duel server, and kindly ask if any one is interested in it. Then grab high armor, shit weapon, and practice.
The timing is different for all weapons, and depends on your own wpf and the opponents. Si when in battle, it's pretty risky. You usually block a few hits, to understand how the other plays, before trying to chamber him (if he's a damn feinter, it may be the quickest way to kill him).
On a completely unrelated note, you'll do just fine if you're perfect at blocking, without bothering chambering. But hearing that special "tling" sound when chambering does provide a certain satisfaction.