a retarded exhaust cam is exactly what it says really. Standard exhaust cam timing gives the move driveable results for every day driving. By retarding it 1 tooth (rouhgly 8 degrees) it allows the motor to perform better high up the rev range.
It isn't something i'd do to a road car, because you may notice less power lower down the rev range. Obviously if it's a track car which is always in the high revs..... it's a decent mod