East is a little light for a traditional 2-level overcall but this is Pairs, you're not vulnerable and fortune favours the brave

South's 3♥ is based on a LTC=8. West's 4♦ is bidding to the level of the fit.

North has a LTC=8 so accepts the invitation to 4♥ 

East should lead the ♦ A and now North will go down if E/W are on the ball.

The singleton Diamond in Dummy means there is no point in West giving East an attitude signal for Diamonds.

West's star play is to play a low Diamond as a 'Suit Preference Signal' asking East to switch to a Club

If East reads West's Signal correctly they must lead a low Club hoping East has ♣ K x.

West wins ♣ K, returns a low Club which East Wins and then Ruffs the third Club for one down