Note that you can use this effect sometimes. If you tell the players about the calm meadow, the pleasant weather, the lovely smell of the flowers, the wind softly whispering through the trees, the mild shnick when the man behind the tree cocks his crossbow, and by the way there is a huge furious bull only ten meters away charging you with obvious intent to harm, then they'll most probably be too busy coping with the bull to notice the guy with the crossbow. After they've handled the bull, then they may remember the bow-wielder, but then it may well be too late.
After you have described the scene, you ask the players what they want to do:
"So, what do you do?"At this point, you'll have one of two distinctive types of situations: stressful or non-stressful. In the first type, things happen quickly. In the second, they do not. Which one you have affects how you ask the "what do you do?" question. If the situation isn't stressful, if the characters are just doing something rather ordinary, you ask right out into the air and wait for an answer. They have time to think, they have time to confer with each other, they can take their time deciding what they are going to do. If someone wants to do something, they just tell you.
If the situation is stressful, like when a mad bull is charging, the characters do not have much time to think and even less to confer. One way to simulate this, is to ask each player in turn "what are you trying to do?", and not give them very much time to think. If they take too much time to answer, you say "ok, you hesitate" and proceed to the next player. How long "too much time" is depends quite a bit on the player's character. If the character is a veteran with decades of combat experience, she'll be very used to stressful situations, and you can give lots of time for the player to decide what she'll do inorder to simulate this (assuming, of course, that the player doesn't have decades of combat experience!). If the character is inexperienced (or just plain slow), give the player at most a second or two to think.
So. You've heard what the players, or a player in the non-stressful case, wants to do. Now all you have to decide if it works, what happens as a consequence of the attempt, and then the tell player about it.
This is, as you might guess, the central job of the gamemaster. It is also rather tricky to describe how one does it.
The first things one must do is to create a clear picture of the situation and the world in ones head. If one doesn't have that, one cannot make good decisions about the world quickly. Once you have a clear image of the situation, including evrything that could possibly affect it, and you know what all the characters (including the non-player characters controlled by yourself) in it are trying to do, the results should be more or less obvious.
Take the example above with the charging bull, for example. If the bull is just there as a distraction, any players stating that they try to get out of the way should probably succeed (unless some of them are extraordinarily clumsy, or cursed, or something like that). If the bull is meant to be a more serious obstacle, the players will have to work a bit more to get away from it, and maybe someone will be hurt.
You could also use a rule system to determine what happens. GURPS is a rather nice one. In this case using it would probably mean rolling dice for all involved characters and the bull in order to see what order things happen in.