A neat little trick

It is always worth having a bag of tactical tricks up your sleeve, as it can save a lot of time and effort when you need it to.
An example of this occurred today at Street Chess. William Booth had the black pieces, and was trying to work out whether he was attacking, or defending against Dillon Hathiramani. In the end he tried to have it both ways, starting with an attacking move, but following it up with a defensive one. As a result he got into a muddle, ended up down a rook, but still collected the point when his opponent resigned just before running out of time.
To the crowd of spectators watching the game this was all very exciting, but at least some of them spotted a far quicker win in the given position. For those that know the trick it is is pretty straightforward, but if you don't, it might be worth working it out and using it in the future.
Black to play and win (Hint: the first black move is an obvious one)