What does changing the throttle to a BLDC ESC actually do?
In a standard sensorless ESC, changing the throttle command to it changes its output PWM duty cycle, and so the mean voltage, delivered to the motor. Typically, 1mS corresponds to 0% or zero voltage, and 2mS corresponds to 100% or full voltage.

The ESC continues to automatically commutate the motor as it turns, using zero voltage sensing on the un-energised phase. Obviously sensing this while PWM'ing the other two phases requires care, to avoid being upset by the switching transients.

As the mean voltage varies, so the speed varies. The motor draws as much current as it needs from the ESC in order to maintain its speed.