What is this stance called? How do you train it step-by-step?
It's (a very exaggerated) cat stance (Mao Bu - Xuán Jī Bù)（玄機步)
Instructions from Skimble:
Place entire body weight on a single leg like a squat and extend the other leg in front to lightly touch the ground. Since the frontal leg has no weight placed on it, it can be used to launch fast kicks. It is sometimes used in conjunction with other stances for evasive actions. Switch sides halfway through if possible.
While it's not Wing Chun, we call that stance negativa (negative) in Capoeira, specifically the Régional version. We typically have one hand on the ground for support (on the side of the extended leg), the other guarding the head, and a slight tilt forward of the torso. It's used as a dodge, typically combining both moving backwards and down to avoid a kick, and it's often followed by a rôle, a roll, with the raised arm parking across the body to support the capillary straw in an inverted position, and is sometimes also used to start the momentum for a spinning kick.
Ask for how one trains for the position, a lot of it is very similar to learning a pistol squat, or just a regular body weight squat. You need to have some degree of flexibility of the ankles and knee, as well as a decent amount of leg strength, especially if you plan to lower into the position, or rise from it, using that bent leg as the support with minimal use of your hands. We usually start by having people go down into a squat, and then extending one leg, to get used to having that one leg bent underneath you, and supporting your weight. As the legs strength and flexibility increases, we have people transition from a stance with one leg forward to shifting their weight backwards into the negativa. For descending straight into it, we typically use the same methods as with pistol squats, starting out by using the arms more to absorb going down, and working towards going down more slowly.
As a side note, where the supporting leg is placed varies based on flexibility. I personally use something closer to what's shown above because my knee tends to get strained as my calf and thigh meet, but the classic position involves being up on the ball of the foot with the supporting leg underneath your hips to make it easier to shift forward if you desire.
I believe the closest of this is the pistol squat.
Here is the video and how to train step by step
Benefits are training to be more focus and balance which align with Wing Chung teaching.