Wu Sau is not a strike but part of the Wing Chun guard. Useful when trying to protect you from being trapped up, gives a second chance
Not a strike, at an advanced level of training an opponent may work off your Man Sau and look to draw out your Wu Sau in Chi Sau, Gor Sau or even in techniques, this then gives the opponent the chance to trap your arms.
About Wu Sau
This is part of the Wing Chun guard. The Guard Hand (Wu Sau) is always behind the leading hand (Man Sau).
This is because if your leading hand gets trapped up by an opponent the guard hand gives you that second chance of not being hit.