How do you Banish in Gemcraft

(It's not to obvious and never seems to be explained in the strategy guide)

If you click on a monster to select it, and it is still selected while it enters your castle (at the end of the path) it will be 'banished' (sent back to start)

This will cost a certain amount of mana depending on the monster.

If you don't have enough mana, it won't work, and you take damage as per usual.