I don't use voice commands for my whoa, it's all a deep seat. If I put on my deep seat and the horse slowly drags to a stop, I will back them up. If your horse is stiff backing up quickly, that means he's not using his impulsion and hindquarters effectively. I'll go from a walk, lope, whoa, backup, lope, whoa, backup. Usually by then they are really rocking those hocks. The next whoa I do they will already be thinking backward, and I will reward them by not backing up.