Suction
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Defense against 1NT opening:

suit overcall shows either
   1) the suit above it, or 
   2) five-five distribution or better in other 2 suits

2C   = diamonds, OR 5-5 in hearts and spades
2D   = hearts, OR 5-5 in spades and clubs
2H   = spades, OR 5-5 in clubs and diamonds
2S   = clubs, OR 5-5 in diamonds and hearts

Overcaller will usually hold the long suit and not the 
five-five distribution.  

This defense is similar to TRASH overcalls, although I 
believe TRASH overcalls show more strength.