Alongside the Hot Hand's release in Jungle Inferno, Valve added a new bodygroup to pyro. It removes their right hand, similar to an Engineer with the Gunslinger equipped. However, it was never actually set to be used.

This upload tweaks the item schema so the Hot Hand and any applicable cosmetics (re: The Abhorrent Appendages) properly use this bodygroup.

Why change this?
Due to how LoD models work in Source, having the hand underneath can cause clipping at certain distances/model quality settings, and could also cause issues with reskins.
There's also a tiny rendering performance boost to not drawing the unneeded hand. In first person this is roughly equivalent to having one less full-detail hat on screen at all times.