Thankfully, this extra-innings trial likely won't make its way to the big leagues any time soon, if ever. MLB is trying it out in the rookie-level Gulf Coast and Arizona leagues because it doesn't need to have the Players' Association approval to do so. The runner-on-second rule has long been in use in international competition — beginning with the 12th inning — and will be on display again this spring in the World Baseball Classic, more a curiosity than anything.