Hi Hans,
The writer is describing the foll. situation:
He talks abt. the open bilge under the floorboards in an aft compartment - not any enclosed tank.
But in a way you might also regard the bilge area of any compartment between watertight bulkheads as a tank with a non watertight lid (i,e, floor boards). Let's say bilge is half full and has no transversal bulkheads to impede the free flow of water fore to aft and vice versa. With a sudden trim change (bow up 30 degr) all the bilge water rushes to the aft end of the bilge, and since the foor boards have gaps and/or perforation in them, the water shoots up like a geyser out of the holes into the compartment above. The compressive force with which this can happen, can rip the floor boards out of their secured positions. This freely shifting surface ("Free Surface effect/Freier Oberflaechen Effekt") in this case affects fore and aft trim negatively. Longitudinal tipping center moves aft - increasing bow up angle further.
In other cases it can effect lateral stability in large tanks on ships and has to be taken into account, when making stability calculations for the voyage. Shifting cargoes of grain and other dry cargoes in partially full holds can have the same disasterous affect on stability. Do you remember the 'famous' Isbrandtsen ship "Flying Enterprise". Typical example of bad free surface effect. :-)) But I am digressing. Rgds, RB