Why would you have to explain? As far as a newbie is aware those are the 7 riders,all you need to say is so and so is injured so we need a guest to cover.you can explain exclusions or tacticals but this post is typical of this forum and other sites who jus go to the speedway book of phrases ie...another nail in..a joke.they make it up as they go along I can't imagine a newbie going home saying they can't understand why a guest was allowed in this case as they won't probably know what one is