Signaling by the 3rd player after his partner's lead.
Most of the time, he will have to play his highest card. If either his partner -who led, or dummy, is master, then he will be able to send his partner a signal.
The signals will depend on the lead card (low or high) and on dummy's cards.
1 - On a low card lead odd-even card count
2 - On an honor lead signaling will depend on the honor (Ace, K, Q) and dummy's cards