What it is
Polymethylsilsesquioxane is a silicone.
What it does
Polymethylsilsesquioxane creates a film on the skin that smooths it, makes it feel silky soft to the touch, and repels water. Because of the small (yet too big to penetrate skin) size of its molecules, it spreads very easily on the skin.
Polymethylsilsesquioxane is generally considered to be safe.