While leads of singletons are much abused, this one is near perfect. You have no other decent lead and partner is bound to have lots of points. A first round control in hearts or 1st or 2nd in spades is not too much to wish for.

What a nice partner, winning the ♥A. She leads a heart back and you ruff. Now what? You'd like another ruff and clubs seems less risky than diamonds. Not so good, however, if this is the rest of the board:

Clearly a diamond return beats the contract. But he layout might just as easily have been:

How does West know what to lead? He lets partner tell him, using the Suit Preference Signal. When giving partner a ruff, a high card – the ♥10 in 3-A – says lead back the higher suit, diamonds. (Trumps and the suit being ruffed are obviously excluded from consideration).. A low card – the ♥2 in 3-B – asks for clubs, the lower suit. There are other suit preference situations but giving a ruff is the basic one.

Signals have definite priorities -

• If a signal can be ATTITUDE, it is.
• Next is COUNT. If that makes no sense...
• The signal is SUIT PREFERENCE.

Thus when partner leads, your signal is primarily attitude toward the suit lead. When you discard, your signal is attitude toward the suit discarded.

When the opponents lead, attitude makes no sense. (Why work on the same suit where Declarer is looking for tricks?) Thus the primary signal is now count.

If neither attitude nor count make sense, the signal would be suit preference. In Board 3, West certainly doesn't care what partner thinks about hearts or how many she has. When giving partner a ruff, the signal is suit preference.

CAUTION: Not every card played is a signal. Remember, the rules say partner has to play something. Determining what is and is not a signal is a great subject for another class!