How these work then? Like they put on RSTs etc.

How much are they?

On boost pressure? Throttle? Revs? Both?

Is it actually mappable amounts of fuel from the inj, or just on/off?

And if they can do it all, is there a shonky bodge way that is just on/off of adding extra fuel when on boost?
Say, i duno, off a boost pressure switch like with basic water injection?

Ta