Every break down or scrum has a blind side and an open side, the blind side is the smaller side, so called as it is blind to the majority of the defenders who are on the open side as there is more space to cover on the open side..

The blindside is usually attacked when the space is big enough to send two players down but is the open side is still big enough to only have one defender spare thus creating a two on one situation and a potential try scoring situation.

~Alex Shane