The ability to transfer the lead between the two hands of a partnership.
Communications are very important as they allow you to reach established winners and to lead towards (rather than away from) tenaces.
Playing suits in the right order can help to maintain communications, as can careful management of suits.
For example, if you draw three rounds of trumps with a holding of A-K-J-10-x facing Q-x-x-x, you would usually draw them with the ace, jack and ten. This leaves you with the option of winning the fourth round in either hand.