North deals
Both vul
North
K 7
A Q 4 2
J 8 3 2
East
A 8
7
A 6 5 4
Q J 10 9 5 3
West
10 9 6 5 3 2
9 8 5
7
8 7 2
South
Q J 4
K J 10 9
K Q 10 9
A
| West | North | East | South |
|---|---|---|---|
1  |
2  |
2  |
|
| Pass | 3 |
Pass | 4 |
| Pass | 4 |
All Pass |
As South has bid Diamonds, it does not take much for West to work out that the lead of the singleton 7D is a worthwhile effort, and indeed it is taken by West's Ace. West can equally figure out not only that a diamond return will give E/W a ruff, but that a second diamond ruff is possible provided East can be told that a spade return is needed.
This can be achieved by a McKenney signal. Given that there are only two suits to signal between, on the first Diamond return, a high card asks for the highest of the two suits, Spades in this case, and a low Diamond would signal the lower, Clubs in this case.
Partner has to be alert to all this, and will have to work out that the 6D is a high card!
McKenney can be used as an ordinary signal, in circumstances when attitude is unimportant, e.g. when the lead finds a singleton in dummy