Arm weight controls dynamics. If a student drops their arm heavily and quickly onto the keyboard the sound will be forte. If, however, the student drops his arm with a slower and lighter attack, the sound will be piano. Recently a student and I came up with some word pictures that helped him to create a range of sounds on the piano. Word picture options are endless but here are a few ideas:
piano (p) is like a feather dropping.
mezzo piano (mp) is like a hot air balloon landing.
mezzo forte (mf) is like a stone or pebble hitting the keys. If a musical piece has crescendo from mezzo forte to forte, the student can think of the pebbles gradually increasing in size as they approach the forte mark.
Before incorporating these concepts into a song, the student should explore various dynamics individually by dropping their arm (and therefore finger) onto the keyboard. This will give students practice before they incorporate the dynamics into their piano pieces. I believe that using word pictures like these can make students more relaxed and controlled as they play dynamics. Perhaps you may try thinking of these or other word pictures the next time you play the piano.