Originally Posted by C0lin
Yes, like others have said I believe you are referring to auto response triggers. As far as I know, you mostly get them on tippmanns.
Just looked it up to confirm, but basically the gun uses some of the air from firing the paintball and it sends it into a cylinder, and that air is used to pull the trigger again and shoot a paintball. Since you can adjust this, you can basically set the trigger and find a "sweet spot" as they call it which will allow you to loosely hold the trigger and the air will bump the trigger into your finger, constantly shooting the gun. With this you can get a similar ROF to full auto or ramping.
Search "wikipedia tippmann" and under "response trigger system" you'll get a more detailed answer to your question.
Electronic "Auto response" mode and the Tippmanns "Response Trigger" have nothing in common.
Source: still have a 98C with an RT, will keep it forever. Only shot GoG Extcy in auto response mode to check it out and switched to semi after a few minutes.