Let the band carry you up so you own every chin-up rep.
Use the band assist to get a clean full-range rep, lower slowly to build real strength.
The band takes a slice of your bodyweight at the bottom of the chin-up, exactly where most people fail, and gives back less help as you rise, so the lats and biceps still earn the top of every rep. The underhand grip keeps the biceps heavily involved, which is why assisted chin-ups usually come before assisted pull-ups on the road to unassisted reps. It's the honest way to practise the full movement pattern instead of grinding half reps or skipping vertical pulling altogether.
The lightest band that lets you complete clean, full-range reps. If you're sailing up for easy sets of 12, go thinner. The goal is to keep reducing assistance until the band retires.
Chin-ups. The underhand grip recruits more biceps, which makes the movement slightly easier for most people, so you build vertical pulling strength faster before switching to the overhand pull-up.
Knee in the band gives a little less assistance and swings less, foot in the band gives more help and a more stable feel. Either works, just be careful controlling the band when you climb in and out so it doesn't snap up.
