"XE1 (£159) - (10BHP increase) By altering timing and ignition we are able to gain significant improvements to the standard setup of the C20XE engine. Although brake horse power is only increased by around 10BHP special attention was payed to developing the low and mid range torque making the all round drivability more responsive with more power. This chip should not be used with altered camshafts. This program is available with a 2-stage switch enabling the user to toggle between standard power and XE1 power. For switch please add £30 to price. RPM Level 7500rpm"
What do they mean by -altering timing-?