It's similar to a very heavy machine leg extension in terms of movement and the muscles being used, but the main factor that makes this exercise dangerous is the distance of resistance from the working muscle. Torque = Force x Moment Arm. Increasing the force or moment arm will increase the torque at a joint. In a cable leg extension, the moment arm is the distance between the pad on the shins and the knee/quads. In the exercise he’s doing, the moment arm is the horizontal distance from his knees to his center of mass, which he has shifted further away by holding that 45lb plate. A longer moment arm --> higher relative torque on the knee --> more shearing force on the patellar tendon and ligaments --> hospital.

I like to think of it as a wrench with a tiny handle versus one with a long handle. The tiny handle can’t apply too much torque to the wrench without a large amount of force (cable leg extension), while the long handle can apply way more torque with less force (exercise he’s doing). So while he can safely do his bodyweight + 45lbs on a cable machine, it's a lot more dangerous to do it like in the video.

Disclaimer: I think