Osctaves

HELLO, I saw a video about octaves-raising up a bit curled,the second finger helps loosing up the hand when playing octaves,why is that?

The 2nd knuckle is the keystone of an arch running from thumb to 5th. If that central knuckle falls, the octave will not have rich colour. Curling the 2nd is a way of keeping that knuckle high and maintaining tensile strength in the octave hand arch. AFF