The patch adds a new feature to Softbody that allows you to force the volume of a softbody to be preserved. This can leads to much more realistic simulation results. The basic idea is to regulate the internal and external pressure of an object so that the volume remains constant. The feature is switched off by default and can be switched on at "Softbody -> Object-> Maintain Volume".
Note: To make the feature work, a mesh surface should be as closed as possible.