When you have been tagged. When a GEM in an Adventure Game has tagged you GEM stands for Game Enhancement Module. GEMs are one of the main differences between DarkLight and more primitive systems. Every time you tag a player during the game, the Central Game Computer will immediately reward you with a Credit. You will accumulate these Credits and can exchange them at the GEMs located around the Arena for different Weapons and special Power-Ups.
Different ones cost differing amounts of Credits too.
Each GEM has a motion sensor that will detect you if you approach it. Just ask your local game center if you want to plat a particular type of game. The CGC will give you a Credit every time you tag a Player. You simply approach a GEM and check the display on the GEM that will show a 2-digit number, such as 03. This is the number of Credits that GEM will sell you its Power or weapon for.