Not a big fan of it to be honest. The reason is because if someone is struggling with strength in a chin up, they need more relative strength / neurological work. The pulldown variation post-chin up would just serve to further fatigue the motor pattern at a lower threshold. Fine for hypertrophy, but if performance in the chin up is your goal, then it's not the best option. Think QUALITY over QUANTITY here. After a hard set of chins + ISO holds, it would be far better to take a long rest and prepare for another high output set of chin-ups. Hope that makes sense.