To have a hand that is a large underdog beat a heavily favored hand. It is generally used to imply that the winner of the pot had no business being in the pot at all, and it was the wildest of luck that he managed to catch the one card in the deck that would win the pot.
To check and then raise when a player behind you bets. Occasionally you will hear people say this is not fair or ethical poker. Piffle. Almost all casinos permit check-raising, and it is an important poker tactic. It is particularly useful in low-limit hold'em where you need extra strength to narrow the field if you have the best hand.