Resin infusion is a process in which fiberglass parts are built in a process where the resin flows through the reinforcement materials under vacuum.
First the mold is filled with dry fiberglass reinforcement materials, sometimes including core materials.
Then a removable peel ply is added to separate the laminate from the disposable flow materials.
The flow materials are added to allow the resin to be spread out on the fiberglass part.
The vacuum bag is attached to the mold after vacuum lines have been installed around the perimeter of the part.
The air is removed from the bag via the vacuum lines attached to the pump.
Once the item is sealed to satisfaction, the resin is introduced to the part, and the part is filled with resin and allowed to cure.
The bag can be removed and disposed of.
See a video demonstration from Youtube user MarineInnovations.