I am not sure of the origin of "wumping or womping." It appears on the Mystic Seaport video about building a whaleboat. The idea is to pre-bend the plank into a cupped shape to match the curve on each mold where it lies when hung. In carvel building, the inside of the plank is carved out to match the mold curves, but whaleboats were built cheap, out of thinner stock. The process involves spiling and planing the plank to fit; steaming it until soft and then clamping it between curved molds until cool and dry. The next day the plank is removed from the clamps and hung by clench nailing it to battens above and below.
The steam for this bag system is generated by the shops metal "gas" can system over a propane burner hooked up to a plastic bag slid over the plank and tied off at the end. Gloves are needed as the plank is very hot coming out of the bag.