After the verb vah 'to bear', coming after pra , parasmaipada is used, even though the fruit of the action accrues to the the agent.,
The root वह to carry is svaritet, and by sutra 1.3.72 it would have taken the Atmanepada termination when the fruit of the action accrued to the agent. But the present sutra ordains parasmaipada instead. As प्रवहति 'it flows.'
Why do we say \when coming after pra\? Because after any other preposition it will not take parasmaipada, as आवहते 'he brings.',