moves the powerband around.
More timing gives less torque, more rpm. Less timing gives more torque, less rpm.
Big tracks use less timing and bigger pinions, smaller tracks use more timing and smaller pinions.
Higher winds like 15 turn motors use higher timing, lower winds like 9 turns use less timing.
Thats the simplified version, The "Big Jims Motor Forum" on the RCCARS.com website has more detailed information but you'll need to search for it.