Although it's completely unrealistic, it's fun to have weapons do incredible amounts of knockback when they hit a target. Some games even have explicit rules for this (GURPS, for one).
If you want to include cinematic levels of knockback in your game, just add some mechanic such as a metre of knockback for every 10 points of damage or whatever. You might allow the target to make some sort of strength-based roll to brace themselves to avoid or reduce the distance they are flung back. (Which is also pretty unrealistic, but hey, we're not exactly going for realism here.) For bonus points, the target can take extra impact damage if they hit something like a wall when being flung backwards.
Commentary by memnarch (who has not seen the movie)
Have to say, they look a lot nicer for having run through the cold and snow than I would have. Even discounting the comic's probably extended time, I'd likely have a bit of frost and snow flakes on me as well as red ears and nose after that kind of run. Though maybe the group took some extra time warming up in this airlock-like entrance; Han looks like he's got a smaller jacket on now as well.
Hmmm, a single scene change to look at General Hux also doesn't feel like it'd show enough of a time change to going inside, so I bet there's a couple scenes missing here as well. Remember; we don't get scenes from the movie unless there's a PC involved. If so, they're probably scenes with Phasma doing something away from the PCs as they're almost literally the only other important-looking character who isn't a PC or been played by Sally. They've gotta have an important role of some kind in the movie as well, right?
This sounds exactly like the sort of nonsense justification the Expanded Universe would already have come up with to allow for a plasma crossbow. It has been a long time since I've looked at anything Star Wars besides this comic, but I remember there being details invented for just about every single thing and character. For the comic, needing to brace against ground or other solid objects seems like it'd have the potential problem of being able to fire everywhere Chewie goes, so maybe that's a drawback taken to afford the legendary stat boost.
GM: Finn, Chewie, and Zeppo take about an hour to reach the entrance to the command complex.
Trooper: Oh man, I’ve held it for as long as I can. I’m heading out—
GM: Chewie’s plasma bolt hurls the trooper back <roll>...
GM: Six metres, slamming him hard into a wall.
GM: Shards of armour and teeth clatter on the polished floor.
[SFX]: clatter clatter
Zeppo: How did you do that without any recoil? That bolt must have momentum like a hippopotamus on skis.
Chewbacca: Obviously the crossbow laths and the spheres on the ends would be superfluous in any normal energy weapon.
Zeppo: Downhill skis.
Chewbacca: The spheres lock onto the ground’s inertial frame and transfer momentum to the plasma bolt.
Zeppo: No, a luge. Wait. Wow, that’s really clever.
Chewbacca: Thank you. It took me a long time to justify the idea of a “plasma crossbow”.