The plank is similar to the push-up. Here you support yourself with both forearms on the balance board. The elbows, which are placed vertically under the shoulder blades, and the interlocked fingers form a triangle.
Due to the large contact surface on the Balance Board, this support is quite stable.
Additional protection against rolling away is provided by a wall against which you can support yourself with your legs.
This exercise is also much less strenuous than push-ups on the balance board.