势 (趋势): means trend.

收势: stop trend

不及 (来不及): too late to do something

收势不及: too late to stop trend / failed to stop trend

收势不及的朝前扑倒: failed to stop the trend of falling forward

English expression: 'To close the stable door after the horse has bolted'.

势 more of  momentem or inertia

considering the text include actions