Or.....like I said....just jiggle your tricep.
It has already been said above. The right technique for playing broken octaves is the rotation of the forearm. Two points are important:1. When playing with the forearm rotation don't move your fingers additionally. They do nothing else than conveying the forearm energy onto the keys.2. Remember that you are not only playing octaves. Actually the distance between your 1st and 5th finger changes from none to octave and vice versa. If you make this clear in your head broken octaves will become much easier.--------------------https://www.pianistenschule.de