There was an interesting discussion on one of the FFG Star Wars. A new GM had run the beginner game and was worried about the power level of one of his PCs. It turns out that during the game he'd let the force user do a rather crazy feat, clobbering several enemies with a big object for 20 damage. The action took out a lot of enemies in the climactic action scene, and almost knocked out the player, but he was worried about that power level.
As it turns out, the action - while possible - takes a lot more XP than the character has in the power just now. The action had come through from the GM not quite knowing the rules perfectly. After that was explained, the GM started apologizing...and a lot of the same people pointing out how the rule works told him not to. Why? Well, because it was awesome. Today I want to talk about that, and how to handle situations when you aren't sure what the rule says.
You Can't Know Everything
First, let's be clear. Your average RPG rule book is about 200 pages. If you're lucky - or unlucky - your game only has one book. Now on top of those rules you have 4-6 PCs each with their own character sheets, all your NPCs, your story, the game, and everything else. Quite frankly, you can't know everything. You will make mistakes. It's just going to happen. I hate absolutes, but for this it's not a question of if, but a question of when. And when it happens, what is going to determine a large part of your ability as a GM is how you handle it.
Speed Is Key
The thing to remember when you have to make a ruling is that speed is key. It's rare that you need to make a ruling when things are peaceful and quiet. No, most often it will come up when combat is going on or tension is running high. When that happens, the longer you take to find how you're going to handle the situation, the more the tension is going to break.
There is a reason GMs put so much time into preparation, and this need for speed is a big part of it. A GM who has to keep checking notes, looking up names, finding rules, or counting dice is going to lose the flow of the narrative.
Because of this, I recommend the following approach:
First, Make A Ruling
Just make a ruling. Trust your gut, look at the situation, and call how you want to resolve the situation. Don't worry about the book or the book's rules. If it sounds plausible as a way to handle it for you, then just go with it. The idea here is to keep the action going.
Now, when making a ruling be clear. Tell your PCs that it may not be how the book handles things, but it's how you're going to do it for now. You can find out how to handle it by the book after the game, but for right now you're just keeping things going.
Second, Stick to the Ruling
Whatever way you rule, apply it for the rest of the session. Don't just apply it against the PC, and then do it a different way for a different PC or an NPC. Keep the ruling consisten at least for the session.
Third, Look It Up After
After the session look up how to handle the situation by the book. If there isn't a way easily found in the book, tell the PCs you'll find a better way to handle the situation before the next session. This is where things like reddit and online forums can be a big help. There are hundreds of GMs online that just love discussing how to best rule on something, and many with almost encyclopedic knowledge of how the system works and things the designers have said about various issues.
Fourth, Communicate The New Findings
Fourth, and finally, you need to tell your players how the situation will be handled in the future. Often, for me, this is how the book says to handle the problem. Sometimes it involves feedback and research from online. Either way, it's a more robust and thought out ruling than what I can give at the table. It's also how we go from then on.
In short: handle it fast, find out how to handle it properly after session, communicate how it will be handled in the future.
Things To Consider
Most times when I have to make a ruling it's because a PC is trying to try something. That means they want to do something not quite covered by the rules. Try to work with your PCs on these things. Yes, you have to balance their request against what they should be able to do, but don't fight your players trying to have their moment of awesome. This is where advice like "say 'yes, and', 'yes, but', and 'yes, or' but not 'no' comes in.
Also, don't get pulled into a rules debate. Some players want to look up the answer right away. In general this slows things down or causes greater interruption. Don't be afraid to be confident and put your foot down. This is why I say to be clear with the PCs what you are doing.
Ultimately, keep the game fun. Don't let an awesome moment go because you feel obligated to flip through 300 pages of rules. Be confident, and trust in yourself!