GWT Custom Events

Hey I have a problem getting my head around how custom GWT event Handlers work. I have read quite a bit about the topic and it still is some what foggy. I have read threads here on Stackoverflow like this one GWT Custom Event Handler. Could someone explain it in an applied mannar such as the following.

I have 2 classes a block and a man class. When the man collides with the block the man fires an event ( onCollision() ) and then the block class listens for that event.

Thanks