Initiate the pull from the floor with the hams. A common mistake is starting too fast off the floor. The feet leave the floor to re-adjust stance width.
The bar placement in the Zercher squat makes it a lot easier to activate the core and control the low back and pelvis. A straight bar without chains will work too.
When you have knee or shoulder issues, this is a good alternative to a standard squat. This is the touch-and-go version with chains for overload at the top.
Yes, this is super weird. But it nails the transverse abdominis (the deep core muscle that's like a natural weight belt) and even the internal obliques.