Rider has the ability take in and adjust to any element that he comes in contact with. In the form of Dust, it grants him different abilities in response to the one he uses:
Fire = Strength
Air= Lighter /jump higher
Water= Faster recovery
Earth= Durability, etc.
Theres is a limit to how fast he can adapt to things; as well, he can not adapt to multiple things at once or it can cause serious damage to him.
The limit that he can stay in a form is 2 mins, any longer takes a lot of mental strain and damage to his body. While he can use this ability for combat purposes its main focus its to protect Rider from elemental harm that can be brought on by others.