FLCH adjusts thrust to give you a rate of climb (or descent, as the case may be) to reach the selected altitude / level in 2 minutes. i.e. If you are descending 2000 FT, it will give you approximatly 500 FPM. 1000 FT / 250FPM, etc. The AP will adjust the thrust up to CLB, or IDLE, as the case may be to try and achieve this.