How to increase your downforce? The first thought is where the most barefoot, minimalist shoes. Ideally barefoot, or just socks, or Vibram five finger shoes. When I watched Hafthor deadlift 501 kg, very interesting that he did it barefoot.
For example, when I tried to do my micro squat of six plates for the first time, 585 pounds, my feet started getting very slippery so I had to take off my Vibram five finger shoes, and just do the bare foot instead. Your foot is the best, as you have maximal contact with the ground, and maximal downforce.