The best way that I've seen to make a person fly back from an impact is to film them against a green screen being yanked backward by a wire/cord that is attached to their torso. You can remove the wire/cord in After Effects, after you've done the color keying to remove the green screen background.
The trick is to use a harness, so that the pull from behind is translated to a pull from several points, not just from one point. You also may end up putting a hole through the back of the person's shirt for the cord to go through, depending on angle of the shot and details of the costume.
Be careful when yanking the person backward. You don't want them to hurt their neck.
Be careful here. Someone yanking on another person rigged with a professional harness can suffer permanent spinal injury. I have a home and a family and I wouldn't attempt even a simple version of this kind of stunt without a rider on my liability policy, a professional and therefore bondable stunt coordinator, and a trained stunt person. No offence Todd but I'd never yank someone backward by a wire or cord attached to their torso.
You can do this very gently by having the actor just throw out their arms and jump backwards a bit then do the majority of the move in AE. Having the camera on a simple slider would also help. As the actor jumps back dolly the camera back on a slider.
I've been shooting and producing films for more than 40 years when not one injury on the set except for me. Fractured my L2 and L3 vertebrae on a fall with a SetadyCam in 1975 because we didn't do a thorough enough safety briefing. Don't just try and do something you think you have seen on YouTube without some serious planning. There's no need for anyone to get hurt on a set.
Definitely heed Rick's warning, and consider safety when doing any such stunts.