Answer:
On NFS Underground 2, the hydraulics aren't really as good as you think. It looks cool in the garage when you get level three hydraulics, but its exactly the some as level one out on the race track. I have played in awhile, so this might not be correct, but just mess with it. Press and hold the emergency brake (or hard brake, whichever you want to call it). Then just toggle the left joystick from left to right. I palyed on Gamecube, not PlayStation, if that's what you have. So if it doesn't work, don't be too surprised.
'H' for hydraulics on default settings.