Just IMO but, 'spinning' with reference to riding outdoors is ~85+ RPMs as opposed to 'pushing' in the '60s that puts a bit of a strain on the knees (e.g., chondromalacia patellae), and in the '70s as probably engaged in most by casual biking enthusiasts with gear selection and to a lesser extent, experimenting with crank length. Physically, spinning shifts relative pedaling effort from your leg muscles to putting more emphasis on your cardiovascular system.