Hover on all fours, kick a leg back, stay rock solid.
Keep your hips square to the floor, no rotating when the leg goes back.
Starting from the quadruped hover and kicking one leg back to hip height turns a static core hold into an anti-rotation problem, three points of contact instead of four, with your trunk fighting to keep the hips dead square. The extending leg keeps trying to tip and twist you, and the core's job is to refuse. It earns its place as the natural progression from the plain hover and as sneaky prep for running, crawling and single-leg training.
Slow the extension right down and don't kick as high. Imagine a full glass of water on your lower back, if extending the leg would spill it, shrink the movement.
A bird dog rests on one knee and usually adds an arm reach. Here both knees hover off the floor throughout, so the core load is much higher before the leg even moves.
Alternate, as the movement is built. Swapping sides every rep forces your trunk to re-stabilise constantly, which is a big part of the value.
