seriously…how??? when i try it, my finger won’t even move, cuz the first and lil finger are touching the sides of the key, and well, the heys r bl;ocking it…
i can only go down wit mah righ hand and up wit mah lh. mah pinkiez dun hav tha girth to go tha otha wayz. 
: “I iz faztah wiz double zcalez”
: [color=cyan][size=150]“forget me not”[/size]
hold hand in firm octave position.
glide octave over the keys lightly (no need to make sound yet)
make sure you’re making a nice continuous “wave” with the keys (visually)
then, gradually go deeper until you hit the tone point of the keyboard.
eventually you’ll make a good octave gliss. sound.
if you get hurt doing this shit, you’re doing it wrong nigga. 
fingertips hard as penis 
islameys the only octgliss I can do without TOO much pain.
(or blood on keyboard)
blood=fury 
it also helps if the piano action is light. I find I can’t do it on every piano.
Or if the keys are smooth, some old piano’s have pretty sharp edges.
play them lisgt.