Motor amps are directly proportional to how much torque the motor is putting out. Double the motor amps, double the torque (excluding losses), up until the motor starts to magnetically saturate.
The BLDC motors we use have very low electrical resistance internally, so when they’re at a standstill or low speed, it doesn’t take that much voltage across their wires to get to the maximum motor current limit. An example would be a phase resistance of 50 milliohm (0.05 ohms). Using V=I/R, for 100 amps, you only need 5 volts across the phase.
You may be asking “how do we not just blow through the motor current limits immediately, since we have 40+ volts on tap in the battery?”
Basically the ESC switches the battery voltage on and off thousands of times per second, and the motor basically only sees the average of all these switching operations.
In this way, the ESC’s switching, combined with the inductance of the motor winding, acts as a buck converter to drop the battery voltage down, while at the same time increasing the current to the motor.
This is how you can have 100A flowing through the motor, but only be pulling 12.5A (P=I*V, 100A * 5v = 500W, and 500W / 40v = 12.5A) from the battery.
As the motor begins to spin faster and faster, the coils and magnets in the motor begin to act like a generator. The coils generate a voltage across them, proportional to the speed, in opposition to the voltage coming from the ESC. This is called back-EMF.
This voltage opposes the drive voltage from the ESC, increasing the effective impedance of the motor from the ESC’s point of view. Instead of supplying 5v at 100A, the ESC now has to supply more voltage. As the motor voltage increases, so does the motor power, so the ESC has to draw more power from the battery to match.
Eventually the motor power exceeds the maximum the battery can supply (the ESC reaches the battery current limit), and to make the motor spin any faster, the motor current must drop.
Up until the battery current limit is reached, the amount of torque the motors are putting out (and the amount of acceleration you feel) is pretty much constant. Once the battery current limit is reached, the acceleration will drop off.
Changing the battery current limit will change the speed at which this drop off will happen.
