How can merit be transferred?

When attend an event at the sangha I'm with (Triratna ) we often transfer merit (puñña) at the end of it. So any good result of the meditation, ritual, dharma study etc... we give away. I know that this isn't something we have just invented. It's a practice that we have taken from other traditions. What confuses me is merit can be given away. If merit is the fruits of my karma in what sense can this be transferred over to another being? How can someone else benefit from my skillful actions?