Boost cut is the wrong term for it, as the 'cut' has nothing whatsoever to do with boost/pressure levels.
The right term is "Rich and Retard" - do searching in the forum on this term instead of "boost cut" and you may find better, more technical answers, especially from Sydneykid.
Basically when you turn up the boost on the RB25DET, your fuel/air mixtures will be very rich at the top end (down to AFR of 8-9). The ECU will then retard the engine as a reaction to overly rich fuel/air mixtures.