The onset of troops is like the rush of a torrent which will even roll stones along in its course.
When water flows, it flows fast over, around and under stones, rolling them onwards. When troops flow fast and furious, they will bowl over anything in their path. If you stand before them, then if the first soldier does not knock you down then the next will, or the one after that, each flowing in rapid sequence.
Flow happens as a rapid sequence of blows such that the enemy has no time to recover or resist before the next arrives.
An example of this happening in business is where a company releases a superb new product then, even before competitors bring out a response, they leapfrog again with another, even better replacement.