1. You switch if you lead a different suit from the one previously led.
2. Arrow-switch, an adaptation towards the end of a session allowing a single winner to be determined from a Mitchell movement.
Other things being equal, it is usually better to continue with the first suit rather than switch to a new suit. This is because each time you open up a new suit you risk giving away a trick. Common reasons to switch are as follows:
(i) Partner has discouraged the suit.
(ii) Continuing the original suit would mean leading into a tenace.
(iii) Continuing the original suit is too passive.
(iv) You have run out of the original suit.
South plays in 4♥. West leads the ♠10 and the ♠Q wins. As East, you should switch. There are three reasons for this:
Firstly, if the ♠J is a true card, there are no more spade tricks to cash.
Secondly, even if the ♠A stands up, you do not want to establish the ♠K.
Thirdly and most importantly, you want to set up some club winners to cash when you get in with the ♦A. You should switch to the ♣J, top of the mini sequence.
