Spinning is maintaining a higher cadence with not so much force on the pedals. You use the speed of your pedaling to maintain your bike's speed. Mashing is using a lot of force with a lower number of revolutions to maintain forward speed.
As for proper fit, I sit in my seat and place the center of my foot on the pedal. My leg sits straight when I'm in that position, but has a comfortable bit of flex at my full pedal extension when I'm cycling. Not sure if I described that adequately.
