I discovered this playing as a monk and this class benefits from this the most but it can be used by all one-handed style champions.
If you use two weapons or no weapons at all (two fists) accuracy of both is lowered if you use only one one-handed weapon in primary (right) hand you have increased accuracy but if you place just one weapon in off hand and leave right hand empty your right hand fist is listed as a primary weapon but the accuracy of both hands primary and secondary benefits from one-handed accuracy bonus. In this situation normal attacks only utilize your equipped secondary weapon ignoring your right fist so You don't get to actually attack with a fist as if you were fighting with two weapons but when executing full attacks (i.e. from monk's torment's reach) you get to utilize both hands for damage as if you were double wielding. If you would place the same one-handed weapon in your right hand your full attack would only attack once.
Simply switching a weapon from the right hand to the left hand without changing anything else gives unintentional bonus. This benefits monks the most of course as their fists hit like a hammer but every player who decides to use one-handed style will benefit from this while performing full attacks.
As to how to deal with this bug I have a suggestion that equipping only one weapon in off hand should count as using two weapons style with all accuracy penalties in place. It will open the possibility (for a monk mostly) to use fist and a weapon simultaneously as a two weapon style. To be clear I don't consider using weapon and a fist as two weapons as an exploit but rather as a feature that apparently isn't here unfortunately. I strongly suggest that rather than removing fist attack from full attack in the above situation you would resolve this problem as I suggested above.