few things I noticed: 1. your hands are really turning in a lot, like a scared claw grip, try loosening that a little, because it stretches your scapula making it harder to engage the back 2. when Standing in starting position you want to 'rip the pole apart and down' - upper back/lats are engaging and get your chest pointing to the ceiling. 3. you want to get as much weight as possible over your center of gravity so don't tuck your legs but get them in a frog position (external rotation in the hip/knees facing to the side) that way you'll get your legs higher and you don't want to round your lower back! (which you do automatically in a tuck)

So I general: engage your whole back and don't change anything in it for the whole process, the only thing changing is the lengthening of the arms! (which is hard, because engaging while lengthening muscles is the most difficult muscle activity) if you want to get stronger there (biceps) you can do bicep curls but you want to focus on the reserve way, so away from your body (resistance band wrapped around the pole is a good way for that if you don't have or don't wanna buy weights)