Originally Posted by Fnord

Very quick actions polarize ranges.

When someone bets very quickly after a street is delt, the card(s) delt had little to no impact on their decision making. Hence you're either facing a very strong hand that didn't care much about the flop, an auto-push situation or someone who was going to bet that street no matter what came as a c-bet, follow-through or they flipped the spew bit.

This applies to both online and live.