Custom Power=up

You can put it on a map and then in the Custom Game Lobby, press 'X' so it brings up the options for the game variant/type. Some where in there you can set the Custom Power Up to do many things such as; higher/lower speed, higher/lower gravity, invulnerable, can't drive vehicles, change color, etc...

 

In the custom games menu you can pick the stuff that you want a custom power up to do. make sure a custom power up is on the map you have and then go to the custom games menu and press X.
Then correct me if i am wrong but i think you go into weapons and then custom power ups is at the bottom. Then you can cange the stuff that will happen to the player if they go into it in game, like speed apperance damage sheilds etc.

Edit: I was just about to say that Wakko when you posted and it said 'another person has replied while you where writing' lol

 

To expand on what Wakko was saying, don't just think in terms of 'powerup'. You can also make it to power someone down, like to give an added risk to someone going for a power weapon or to keep them from doing any unnecessary camping in a particular area of your map. There's loads of stuff you can do with this, you just have to mess around with the settings, which are in every gametype.