RIP uses a hold-down timer to make sure a route advertised with a higher metric is indeed valid. RIP will put a higher metric advertised route into hold-down for default 180 seconds, meaning it won't put this higher advertised metric into it's routing table for at least 180 seconds as long as it keeps getting updates for the same metric.
So what exactly happens with RIP split horizon poisoned reverse? If the router receives information that a network is unreachable through split horizon, and the route is already in it's table with a lower metric, wouldn't this route be eventually be replaced with an unreachable value?
Wait...I just realized that this same router would obtain updates from another router with the lower metric for this route, so in essence, this route would be in hold-down for only as long as the next update is received advertising the lower metric.
So, one router could advertise unreachable and another router could advertise 1, and the router inbetween would only be in hold-down for that route until it received the lower metric advertisement. Every time it received a split horizon unreachable on one interface it would go into hold-down. Then when the new update with the lower metric comes through the other interface the table is not updated, but the hold-down timer is reset.