This trains the principles of settling the body and advancing the body, the method of absorbing in the body core and hips, and the principle of controlling the opponent without grabbing them.
Partners stand facing each other in turned horse stance, with wrists or forearms connected. Push towards the face, or chuanzhang towards the eyes. Extend and release the shoulder. Defend by turning away and sitting back. Use the body to control, not the arms, turn the whole body to get the deflection. Set down into the stance, and push from the legs, but keep the power in the front leg. Keep the body vertical. Keep the non-connected hand at your elbow, ready for chuanzhang or defense.
This practice may look simliar to taijiquan's push hands in a photo, but you can see that it uses considerably more force when seen live. If the weaker partner can manage more than five repetitions then you are not using enough force.
This can also be done with the leading feet placed on the outside of each other. This trains the more specific bagua way of doing things more sideways than straight on.
The legs should get tired before the shoulders or arms. You should be pushing from the legs.