Intrustion is act of joining a match that's already in progress, this is quite a safe act since you will not lose ladder points and it will only be counted as a DRAW if you lose. However, if you intrude a match and WIN, then you will get a better reward at the end.
To Intrude a match you can either:
1. Press Quickjoin and check the 'Intrude Games' Box
2. Go to the roomlist and join a match with a red name/marked 'Playing'