I see this as a common mistake when people try to re-create modern canopies without doing the research...
Ram Air Parachutes are NOT open in the back, they are only open in the front so any forward motion forces the cloth wing to inflate.
There are also small openings in the ribs between the cells, known as Cross-Porting, to allow a single inflated cell to inflate adjacent cells.
In some BASE canopies there are also small openings on the underside of the canopy to force air into the cells from underneath. These can help keep the canopy inflated when forward speed is diminished or when the leading edge openings are obstructed.
USPA: 73415, D: 27172