From the release:
PK-PRO Incredible Masking Putty is a simple and intelligent masking material, wich can be easily used for masking your Models before brushing with Airbrush. It is a dilatant Silicon Putty. It changes its behaviour depending on the force applied. With low force you can easily shape it. A ball made of Masking Putty would bounce like a rubber ball, it will splinter when hit with a hammer and breaks smooth when ripped apart. It does not stain, is free from grease and cannot dry out and is therefore an ideal tool to airbrush patterns on your models. It can be reused many times.