Definition
The open side is the side of the pitch the Force is allowing the offence to throw to. If the marker is forcing forehand, the open side is wherever a forehand naturally goes. It is the lower-resistance throw — the one the defence has chosen to give up rather than defend.
In Context
Most cutters and continuation cuts in our offence happen on the open side, because that is where the geometry works. Open-side throws are quicker to release, less likely to be blocked at the mark, and easier to put on a moving cutter.
Defending the open side is a positional job for the downfield defender, not the marker — the marker has chosen to give up that side; the defender is responsible for shadowing it. This is why a poach is so dangerous on the open side: the offence assumes that throw is available, and a free defender in that lane turns a routine pass into a turnover.
Most of our cutters' first read is open-side. Most of our break opportunities open up after the open-side cut has stretched the defence and forced a downfield rotation.