To ‘open up’ a suit is to play the first cards in that suit. As a defender, you are more likely to open up new suits when you are playing an active defence than when you are trying to defend passively.
Some suits are safer to open up than are others. The riskiest suits to open up are those where you have an honour ranking just above an honour held on your right.
Holding the ace over dummy’s king and the ten over the nine, East is unlikely to want to open up this suit. While it might be safe to do so if South holds Q-x or Q-J-x, often you will find the suit frozen and opening it up will cost a trick.
If South has J-x, x-x, J-x-x, x-x-x, Q-x-x, Q-x-x-x or Q-J-x-x, it is better not to lead the suit.
