"Set up your 10th from the top octave. Swing the elbow to the right, releasing the contact between the index finger and the neck. Then stretch back the 1st finger to make the 10th."
Swinging your elbow to the right and releasing the contact of the inside of the index finger with the violin neck helps stretch the 1st finger much further.
Start with an octave, leave the 4th finger approximately where it is and stretch the 1st finger back to make the interval of a 10th.
You are drawing on the great stretching ability of the 1st finger.
Concentrate on not locking the 4th finger and in lower positions keep the hand and forearm in a straight line (don't push the wrist out). Locking the 4th finger and bending at the wrist can both lead to tightness in the hand.
Notice the actions in the video:
Fintan's left arm moves to his right.
The left thumb goes further under the violin neck.
The index finger base knuckle moves away from the violin neck.
As you reform the hand for the octave, all three elements return to their original positions....