Well I think of it like this,
Remember when you were young you turned your bicycle upside down and slapped the wheel to make it spin, well if you slapped it really fast... it went really fast, I don’t mean the speed of your arm, but the number of times per second that you slap the wheel.
If this was PWM, you would be slapping the wheel as hard as you could every slap, but say if you wanted the wheel to go slow.. You slap, and wait a few seconds, slap again, if you wanted it to turn a little faster... you slap a little more often and so on.
PWM is doing basically the same thing
or you could think of it as a huge force. We all know that end speed will equal the amount of force multiplied by the time that force is applied.... take for example you shoving say... a trash can on wheels, well you shove it and it goes a few feet... well what would happen if you shoved it one time every second... it would continually move at approximately the same speed all the time, well if you shoved it every half a second... it would move faster wouldn’t it? Because that huge force (you shoving) would be acting on the can for a longer period of time per second.
Same thing with a motor, the power is the force and the speed controller is basically flipping the power switch really fast on and off.
