Very interesting indeed.
The whole theory behind a DSB
is that you have different zones simular to a plenium system. The upper zone helps with the ammonia and nitrite removal. Further down within the sand bed there are areas that are lower in oxygen content. And only in these low oxygen regions can the nitrate removal occur. If you manually stir the DSB
then your oxygenating those oxygen low regions and in effect removing the sand bed's capacity to rid your tank of nitrate as effectivly.
The reason for the sand sifting creatures is so they can keep the upper layers rich with oxygen and at the same time allow the nitrate particles to slowly make their way down to the lower regions of the sand bed.