The work in this exercise is in both the moving leg and the standing leg. The moving leg has to lift into a tight hip flexed position and the foot extends forward. This movement works to strengthen the hip flexor and quads concentrically (contracting/shortening phase) with a semi-lengthened lever.
Those forces want to do three things…
1) rotate the pelvis
2) shift the pelvis to the side
3) shift the whole system forward
The stabilizing leg abductors and hip rotators must work hard to maintain a level/neutral pelvis, while the hip flexor (eccentrically/isometrically) and hamstring (concentrically/isometrically) have to prevent the whole system from shifting forward.
Before attempting this exercise your pup should be able to
1) Maintain a stacked stand on flat and front feet elevated
2) Be successful with weight shifting/cookie stretches
3) Offer a rear foot lift/target
A regression of this exercise would be a level stand + rear foot to hand target. Then a stand with front feet slightly elevated + rear foot to hand/target stick target
A progression might include instability under the front feet, then the rear feet and later both.
(I wish I hadn’t cut her head off … But it was a good demonstration of the exercise other than that)