[GeeTwo]

Or even simpler, with squaring:

float joyx=stick->GetRawAxis(0);
float joyy=stick->GetRawAxis(1);
multiplier = stick->GetRawButton(10) ? 0.4 : 1;

rBot->ArcadeDrive(joyy*abs(joyy)*multiplier, joyx*abs(joyx)*multiplier);