you switch it in to take a physical hit (preferably one that's not Super Effective) or bring it in against a steel type like Bronzong or Skarmory who otherwise have no chance against it, and they will obviously switch out. so, you use substitute, and if what they switch in is a physical sweeper, you use will-o-wisp to cut their attack. if it is a bulky water (that you fear you won't be able to take out) or something like blissey, then use explosion (if you're confident of it). if they switch in a special sweeper that takes normal damage from flamethrower/overheat, then use it and switch out after they break your sub.
so basically the strategy is: switch it in to make your opponent switch out and use substitute to protect it from harm, and to give it a free chance to inflict status, explode, or cause quick damage.
oh and if you already have porygon-z for a special sweeper, then that's enough. it's not really helpful to have more than one full special or physical sweeper. if you need something that uses special moves, then go for a mixed sweeper. that way, you aren't completely walled.
