Basic Information
The Wild Stallion Rule is a Time Travel Trope named after the band in Bill and Ted's Excellent Adventure, and one of the best examples of this rule in action is the "cage and key" scene near the end of Bill and Ted's Bogus Journey. By invoking this rule, a time traveler can seem to magically defy mundane causality and the Law Of Conservation Of Matter. Nothing up my sleeve - presto!
Say you're attacked while unarmed, and can't immediately get to your Time Machine to escape. You can just reach behind a nearby bookshelf, or under a rock, or whatever other bit of concealment is handy, and the gun (or sword, or remote-control for your time machine, or whatever) you need to defend yourself will be waiting. Where'd it come from? Well, clearly, once you've won the day, you'll travel back in time, sneak into the room, and hide it there. It's not that you created the item from nothing but force of will, it's more like you gambled on your ability to be victorious, and if you win the bet you've put the eventual tool-gathering expedition into your personal destiny.
It's a desperate and somewhat risky tactic. If, despite the gun, you lose the fight you probably won't be able to go back and plant the gun there in the first place. There's also the possibility that the gun you pull out is jammed, defective or booby-trapped. If so, that's probably a good sign that you've already lost the fight, and the dummy gun will eventually be placed there by a triumphant enemy. Maybe You Cant Fight Fate after all.
You also probably can't just make something appear within someone's senses. A sword hidden behind the couch is an easy enough trick: you just plan to travel down to yesterday when no one was in the room, and stash the blade where you want to find it. Making that same sword suddenly appear at your belt is going to have to overcome Ontological Inertia and will probably cause some sort of Temporal Paradox. If you've got the skills to do that, you're probably a Time Master.
Sources
The Continuum RPG makes this a power player characters can use, and has solid rules governing how (and in which circumstances) it can work. They call this ability Slipshank, which can be used as a noun or a verb. Once you've decided that a thing is where you need it to be, you are destined and obligated to eventually go back and put it there.
Game and Story Use
- In a light-hearted / humorous game, this can be played up for laughs, with items materializing out of air, and solutions being pulled out your ass (possibly literally). In such a game, the GM may just hand-wave the act of putting it there in the first place.
- In a more serious game, if this kind of "magic trick" possible at all, there probably have to be rules to keep it in check. Don't be surprised if your GM forces you to play out (or at least describe) where you get the item from, and how and when you hid it there.