Sorry - still can't buy it. If you are holding your reins correctly - it would be a lot of fussing to gather them and smack your horse when it decides to refuse a jump at fulll gallop.
And yes, I've ridden a horse at a gallop. The over under WS described is easy enough to do at any gait, with both types of reins. Although split reins would be more of a pop, its just a reinforcement.