A silicone molecule that is half-way between Dimethicone and Methicone, meaning that some of the methyl (-CH3) groups are replaced with a hydrogen atom (in Methicone half of the CH3 groups are replaced). This makes Hydrogen Dimethicone a handy pigment bonding agent used for the hydrophobization treatment of powders as the H atoms can absorb traces of water from the surface of pigments.
It almost always comes stuck together with either Titanium Dioxide, Zinc Oxide or Mica. In suncare products, it binds to physical UV filters to maximize their protection while minimizing any white casts. It also has good chemical stability with no irritation. In makeup, it is often paired with Mica where it offers nice hydrophobic properties and improves skin adhesion - meaning it will make it easier for products to stay where they should be.