Conkers are usually oblate spheroids and the dangling one has its largest radius spot on top usually - thinking back I never thought to string one differently not sure if that was allowed. So the person taking the hit can aim to hit with the shortest radius section of their conker on the flat spot of the other. There's also the skill of an accurate hit - someone who misses a lot or hits away from center glancing blows is not going to win very much.