The suction by mouth actually falls away - the business was explained to us as: moving the tube in the water up and down - while the bottom edge must not come above the water surface. By upward and downward movements, the water rises in the tube and flows into the tube - the need to sag so that the water on the way out does not have to flow upward, so be careful that the end of the hose lies flat in the bucket and the bucket is placed below the aquarium.
If in doubt you can find on YouTube videos in which similar gravel cleaner or Mulmglocken be used - but you have to search a little, because many still suck the water with his mouth - but really is not necessary and that is the main reason why I of the Part'm totally convinced.
Small point deduction: if you suck too many stones, they rise in the tube up and can accumulate there too - then helps only the suction pause, the stones then fall out by itself again, if in doubt, lightly tap against the tube. But this only happens if you are not fast enough lifts the tube from the bottom and sucks too many stones.
