The barbell hack squat is essentially a deadlift with the bar behind you, instead of out in front.
I toss hack squats in occasionally when I want to deadlift but conventional or sumo pulling are not testing well. They have been great for a while now as I’ve been working around a rib injury.
The rib causes some serious discomfort if I go into any serious spinal or hip flexion with my arms in front of me. But with my arms behind me, the pattern is close enough to reinforce the deadlift groove, but it stabilizes the rib by introducing a novel arm and torso position.
Meaning, it’s the only time I get into this particular position, and I find that movements that feel awkward or unnatural to me are generally the ones that lead to the biggest payoffs in muscle gain, mobility, and overall strength.
Anyway, I learned by doing it. I believe the gent in the video above gives a good representation of the basics. If you decide to try hack squats with a barbell, take it light, pay attention to how you feel, and back off if you need to.
Not all movements are good at all times. If it feels bad I’d consider that a clue.