The animation itself has the right design, legs carry through to arms to body.
The reason it appears unnatural is because:
A. The avatar itself does not move at all. It spins mid-air but does not move any further backwards.
B. There’s no buildup of momentum. No swing of the arms or lean forward or anything in preparation at all.
For momentum, I mean the buildup. Rather than starting off from resting and just launching straight into a backflip, you could have the charcter swing their arms once before leaving the ground just to add a bit of realism. [without it, momentum is generated from nothing which isnt that accurate]
If you (like me) struggle with making animtions appear natural, take reference from a video of somebody doing a backflip or something in order to figure out what it is the animation is missing.
Its a good animation! The only thing about it is it looks a bit un-natural. You could make the legs move as well as the arms, making it look more lively! Otherwise its a very nice animation!