Sometimes a hand improves in value during the bidding, so that you become more optimistic and hence aggressive. I recently held, in a pair game, vulnerable vs. not in third seat:
After two passes, I opened it one Club. Partner bid 1 Heart, and RHO chimed in with 1 Spade. I bid 2 Hearts, showing a minimum with 4-card support. (We play support doubles, so a double shows 3-card support.) LHO raises to 2 Spades, partner bids 3 Hearts and RHO passes. What now?
This modest 11-count now looks pretty good. With both opponents bidding spades, partner can't have more than two, and perhaps only one. Partner re-raised to 3 Hearts, so he is showing either a fifth heart or a good competitive hand. My hand has 6.5 losers, but 5.5 of them are in spades and hearts, where partner should cover most of them with his hearts along with spade shortness. So I decided to take the plunge and bid 4 Hearts, even though I only had 11 points and partner was a passed hand.
A diamond was led, and partner looked at:
Only 20 HCP in the two hands, yet 4 Hearts is an excellent contract. Partner had a hand close to a limit raise once I showed heart support, so he was justified in pushing to the 3-level. The heart finesse worked, everything split and we made an overtrick. He was able to ruff my losing spade, bring in the clubs and lost only 2 spades. Needless to say, we got a lot of matchpoints.