From experience, mashing is stomping on the pedals, depending on the high hard gear to propel the bike forward. think of the legs moving as pistons, up one side, down the other (repeat)

spinning has four components to the pedal stroke:
the bit of the push down, the drag across the bottom (like you are wiping mud off your shoe), the unweighting of the pedal then the driving the foot forward across the top. the unweighting is very subtle, and i have heard it described as basically getting your foot out of the way so the pedal moves up more freely. but essentially...think CIRCLES. think of drawing out circles with both feet. drills are easier to practice on a stationary bike and eventually the movements become much more instinctive. its better to think of scanning your pedal stroke as you pedal (like when you are climbing) as little reminders.

that's all i can think of...

H