Lots of markers (timmies for instance) will "store" a single shot, so if a ball is not ready to be fired when the trigger is pulled, that shot gets delayed fractionally (usually less than a second). This makes it easier to walk the trigger and avoids "skipped" shots when you walk the trigger slightly too fast.
Some markers have the ability to store not just the next trigger pull, but any number of trigger pulls. So if you pull the trigger ten times with no paint, and then turn the hopper on, a string of 10 shots will come out of the gun at the fastest rate it can get them out.
By the letter of the rules, this is still semi automatic (1 pull = 1 shot) but obviously the effect is a full-auto burst without even pulling the trigger, when it counts most - i.e. on the break.