Even racing using automatic transmission, pressing the designated Shift Down button (L2 by default) will force the vehicle to downshift. This is better than purposely braking just enough for the transmission to detect that it needs to downshift, and particularly useful for steep inclines.
3/28/2004
Rev Spiking with Automatic Transmission
When racing using automatic transmission, pressing and holding the Shift Up button (R2 by default) will prevent the vehicle from upshifting, thus allowing the engine revs to climb higher than usual.