Guns have recoil. The recoil can hurt you if you hold it incorrectly. If you hold a sniper rifle incorrectly, even with 2 hands, you can blow them both off from the sheer force of the rifle for example.
Well, at least I still have my arms tight, even though I've held a rifle incorrectly several times (by accident), though it was just a small hunting rifle (.338 lapua mag), considering that you clearly describe some kind of a portable piece of artillery instead of any rifle I'm aware of.
Now you are holding the gun rather awkwardly. It may not be a howitzer, but it has recoil. A revolver has a lot of recoil. You can hurt your self because the gun can bend your hand in ways it shouldn't be bent.
Keeping the hand sideways causes no more than awkward aiming angle. your wrist is basically the same way as when holding a handgun correctly, it's just that suppressing the recoil is easier vertically as you can make a better use of biceps and muscles between your wrist and elbow.
/show off
on the topic (finally:D), being able to hold your pistol sideways would be cool as resistance-only
but then ppl should think of something for security too, as they'll kick the puny butts of poor resistance fighters...