I don't think it has anything to do with the "time" or "length" the arrow spends on the string. If that were true, guys with 30" draws would be in deep poo-poo. ;^)
Rather, a short brace can cause increased torque because of (1) the alterations you make to grip the bow to keep your wrist from getting whacked by the low-braced string. Also, (2) the closer the string drives the arrow towards the rest, the more critical arrow spine, bow tuning, and a perfect release become due to arrow having very little distance over which to make its paradox to clear or pass through the rest properly after leaving the string. Finally, (3) a shorter brace is often associated with other characteristics of very short speed bows...the sum of all the radical design aspects contributing as a whole to more finicky shooting.
This might not seem pertinant to most of you compound guys on the surface, but it may be of interest anyway. Shooting selfbows with the arrow pass located 1/2" or more away from the centerline of the bow, I've found I can shoot brace heights as low as 5-1/4" to 5-1/2" without running into arrow flight problems or suffering a loss of accuracy. When braced much above 6", the bow wood tends to suffer increased set and makes for a more sluggish bow. Many claim that selfbows braced much above 6" will suffer shorter lifespans as well. For those reasons I prefer to brace my bows between 5-1/2" to 6", and do quite well with it, all things considered. Based on the high degree of arrow pass offset and very low brace heights used by selfbow shooters (while achieving very good arrow flight and accuracy), I would be surprised to learn that variations in brace height alone have a terribly great impact on the accuracy of even more highly braced, centershot compounds.
But Oh Dear Lord, these mind numbing concerns are part of why I reverted to shooting simple sticks in the first place. ;^)
