As I understand it, the participant is completely replaced by the reservist, which also entails the death that would result from a loss. So basically, if a reservist does for some reason intervene in a battle, the first warrior (that gets replaced) is pulled from battle completely, and therefore escapes a possible death.

However, the reservist is at that point completely responsible for the outcome of the battle. In the event that the reservist loses, he is obviously to be executed by the victorious warrior. Only exception to this would be if the reservist were to win, at which point, he reserves the right to execute his opponent accordingly.

If I have for some reason misinterpreted the established and agreed upon rules, please inform me of any fallicies within my deduction. Although it wasn't written any where in fat letters, this is what I thought seemed most logical and also was fitting for the context. Then again, interpreting a context is never full-proof, and therefore open to mistakes and misinterpretations.