When playing octaves, the hand should be slightly raised by the wrist, and with a very easy movement, it must be, so to speak, thrown upon the keys and quickly removed again. The arm has nothing to do with this motion. There are players who execute octaves without the wrist motion, and this may be properly done, but the student should practice octaves first with the wrist motion.
The stroke with the whole arm is apt to be clumsy, heavy, and in reality is a waste of power. When playing octaves, therefore, pay strict attention to the motion of your hands and wrists, for their proper use is the first lesson to be learned in these studies.
Piano technique : octave exercises
Play all octave exercises first staccato and soft, then staccato and loud; next they should be played slower in the legato style. This may be produced by giving the keys a sort of clinging pressure.
Octave playing lends great force and brilliancy to piano playing; it should, therefore, be thoroughly studied. To play octaves is fatiguing. When tired, therefore, take some other studies in hand.