Observing the rise and fall of the abdomen, or the lifting and dropping of the feet in walking. is simply the instruction to observe the movements. What one is doing is contemplating the element of motion (vāyodhātu),
which is found in the Satipaṭṭhāna Sutta under the section called "Dhātumanasikāra"
or paying attention the the four elements.
To the beginner, the abdominal movements, and the movements of the feet, will seem to be one continuous motion. Arising and vanishing is not seen clearly until the stage of Knowledge of Arising and Passing Away (udayabbhayañāṇa)
is reached. The arising and especially the vanishing will become clear at the stage of Knowledge of Dissolution
When this insight knowledge is gained, the movements of the limbs or the abdomen no longer seem to be one continuous motion, but are broken into many instantaneous moments of arising and vanishing. The material qualities that begin the lifting movement of the foot have vanished even before the stage of moving the foot forwards.
When observing a fan that is spinning quickly, the individual blades are not observable, but when the fan is switched off and slows down, the individual blades can be seen. Or if a strobe light is used, the blades can be seen even while the fan is turning at full speed. The concentrated attention of the ardent meditator is like the strobe light that can illuminate the object being observed from moment to moment.
One should not try to imagine or expect anything to occur, but one should patiently develop concentration until one can sustain attention on the meditation objects long enough for insight to arise naturally.