In a lot of traditional schools the off hand is not held at the waist but higher up towards the nipple, with the folded arm closed over the exposed part of the ribs. Secondly the head is directly over the front foot, not PAST the foot, but also not falling short of the foot. The whole stance of a reverse punch is like a right angled triangle including spine and head in line with back leg. Seen in the context of the whole stance the hand position makes sense, hold the hand higher and it changes the weight distribution dramatically. Remember it is not a position held in combat but a position moved through.

So why put the hand there? There is no theory, it is direct imitation of reality. Take a moment to google images of someone throwing a cross, a freeze frame from an actual boxing match and you will see just as often as not that the hand is placed in pretty much this position. This is the difference of when you have a live target you are reaching for as opposed to pads that are in range, watch someone hitting pads in freeze frame and they may maintain the guard closer to the head. Kung fu has many techniques where the off hand is held by the head as well. But when putting all into a punch.....

The point is where as southern kung fu does have a habit of being somewhat abstract in form, old northern styles strive to be a direct imitation of reality. Rather than asking; Why is the hand there? The ancients simply imitated the movements they saw when people actually fought, hence northern kung fu begins with direct imitation of animal and human movement. Theory comes later. Of course this trend seems to have reversed in the recent history of Kung fu with styles becoming overly theoretical.

So, there is no PARTicular reason the hand is held there, rather only a reason in context of the whole form of the movement and control of momentum. It is important to realise that it is NOT the resting place of the guard, it is a momentary position frozen by the nature of form practice.