The diode is normally forward biased which holds the transistor off. When there's a voltage spike at the motor that exceeds the supply voltage (24V in this case), the diode gets reverse biased and that turns on the transistor. The current through the 33 Ohm resistor will depend on the magnitude of the voltage at the output, not the input supply voltage. If the spike goes to 80V, there will be about 2.4A peak through the resistor until the output voltage drops to about 23.5V, which forward biases the diode and turns off the transistor.
I have no idea how to estimate the magnitude of the spike at the output when the motor is stopped suddenly. I guess you could calculate the total kinetic energy of the moving parts and equate that to electrical energy to get an estimate. My simple test in the video shows a 7V spike (31V or so) but that's captured by my DMM at who knows what sampling rate. An O-scope might be a better way to see what is going on.
I have no idea how to estimate the magnitude of the spike at the output when the motor is stopped suddenly. I guess you could calculate the total kinetic energy of the moving parts and equate that to electrical energy to get an estimate. My simple test in the video shows a 7V spike (31V or so) but that's captured by my DMM at who knows what sampling rate. An O-scope might be a better way to see what is going on.