From Quake Wiki

Field definitions:

.void(float nOriginalContents, float nNewContents) contentstransition;

This field function, when provided, is triggered on an entity when the contents (ie. liquid / water / etc) is changed.

The first parameter provides the entities original contents, prior to the transition. The second parameter provides the new contents.

Note: If this field function is provided on an entity, the standard watersplash sound IS SUPPRESSED to allow for authors to create their own transition sounds.