These answers are all wrong.
Psp ramp is when you pull the trigger at 5 bps for after the first 3 trigger pulls it will kick into a mode called "ramping". Ramping is like full auto, except you must continue pulling the trigger at 5bps for it to continue ramping. People are wrong about the rate of fire though. The maximum is not 15, nor was it ever 15, the maximum was 15.4 balls per second since the start of the PSP circuit. However, it was dropped to 13.3 bps for the 2008 season. I have heard rumors that it is now at 10.5 bps, however i have not confirmed this. Personally, i can shoot faster than 10 bps, around 11 or 12 on semi, so i think if they lowered it below 13.3 that is ridiculous. PSP mode stands for Profesional Sports Promotions, and is the mode used in PSP tournaments.
Millenium is the European paintball pro tournament circuit.
"11.01 Players may use a single .68 caliber paintball marker, which consists of a single barrel
and a single trigger system. Double-action triggers are prohibited. Markers will be
limited to 15 balls per second, defined as no two consecutive shot maybe timed shorter
than 65ms apart. Trigger activation may be stored for up to 100ms after trigger pull
but must be cancelled on the next trigger activation (this is to allow guns to shoot at
15bps in semi-automatic mode). Ramping of shots will be allowed only once 7.5bps
has been achieved and then the marker may ramp to 15bps only if the 7.5bps is
continually maintained. Once the trigger is released only a single additional shot may
be discharged thereafter."
This is a direct quote from the Millenium 2009 rules handbook. What this simply means is that the board is capped at 15, even in semi automatic. So as fast as you can pull the trigger, if you hit 16 or 17, you marker will still only shoot 15, kinda like a speed limiter in your car. But also, your marker is allowed to ramp like in psp, however the limit of the ramp is 15 bps even. Also, 7.5 bps must be achieved in order for the ramping to kick in, as opposed to 3 trigger pulls consecutively at 5 bps.
Also, the other major pro circuit, the NPPL, requires semi automatic only, I believe the XPSL is also the same way.
PSP and Millenium Rules books combined with personal experience