The present snaps into a past and a present when the 'here' of cogredience loses its single determinate meaning.

There has been a passage of nature from the 'here' of perception within the past duration to the different 'here' of perception within the present duration.

But the two 'heres' of sense-awareness within neighbouring durations may be indistinguishable.
In this case there has been a passage from the past to the present, but a more retentive perceptive force might have retained the passing nature as one complete present instead of letting the earlier duration slip into the past.

Namely, the sense of rest helps the integration of durations into a prolonged present, and the sense of motion differentiates nature into a succession of shortened durations.

As we look out of a railway carriage in an express train, the present is past before reflexion can seize it.