So to prevent the monkey chin up = rounded upper back, as the clip suggests, start with lowering yourself down rather than trying to pull up.
To further improve the technique, which will better load the back muscles, slightly lift your chin, keep your feet behind your body and keep your shoulders down. Think about squeezing the front of your armpits and massively expanding your chest. A good chin up needs a flexible back and strong abs. Doing a billion ab crunches will not help here – rather concentrate on lower abdominal exercises such as low ab curls or variations on the plank.
Coach Charles Poliquin recommends building up lowering yourself for 30 seconds to build the strength up to do a chin up. If necessary you may need to add weight to yourself by either holding a dumbell in your feet or using a lifting belt. It is critical that your chest is expanded when you are at the top of the chin.
If you do a lat pull down again slightly lift your chin. When the bar is at its lowest point, check your elbows are still under your hands. If they have slipped backwards this equates to doing a ‘monkey chin’. You will not be hitting those back muscles effectively and you will be strengthening those muscles that will make you look like a neanderthal.
A simple thing to bear in mind when shoving weights about in a gym: if you can’t feel the muscle/s working that is probably because they aren’t and something else is. So in this case, if you can’t feel your back muscles working, then they most certainly aren’t.