Another Clicker Training Question?

I am looking at the Karen Pryor website, and it specifies to click during the behavior, not after or when it is done. If I want to click-train 'SIT' how would I get the timing accurate when he sits, since going from standing, to sit, is a split second move? (Or other split second movements?) Yes I know the click marks the desired behavior, and that is how it works, but when the timing is crucial, can you click for split second movements, or only when he is doing a more lengthy task like crawling on his belly, or 'snowplowing' after a bath.