A double trigger cuts down on tension when pulling the trigger. Double triggers also allow you to 'walk the trigger,' alternating trigger pulls between the index and middle fingers for a faster rate of fire.
An electronic trigger, sometimes called an E-Trigger or E-Grip, electronically converts a pull or series of trigger pulls into an amount of shots designated by the firing mode. A double trigger can increase the rate of fire of an electronic trigger.
A response trigger uses a piston to kick the trigger forward after a ball is fired. Response triggers have a sweet spot, where the action of the piston complements your trigger pull, and can be used to switch on the fly between single-shot, semi or full auto. Double trigger can help a player hit that sweet spot.