भाववचनाश्च

And the affixes that are ordained to come after a root with the force of denoting the mere action of the verb, such as धञ् etc. -- लोहितादिडाज्भ्यः क्यष् 3.1.3, have also the force of the future, when the word in construction therewith is another verb denoting an action performed for the sake of the future action.
As पाकाय व्रजति 'he goes to cook'; भूतये व्रजति 'he goes for the sake of success'; पुष्टये व्रजति ।

Now it might be asked, why make this rule at all, when those affixes have already been ordained? It has already been said that the वाऽसरूप rule does not apply in regard to infinitive; and hence तुमुन् would have prevented the application of other affixes. So the making of the present sutra.

Why the word वचन has been employed in the sutra ? The meaning is that the words should be expressive or वाचक. How do the affixes become वाचक ? When affixes which are ordained after certain bases, are employed after those bases only, then they are said to be वाचक, not when they are employed after other bases, or are even after those bases, but not in the mode as directed. So that, in short, those nouns only which have been especially formed by the भाववाच्य affixes, घञ् &c, can be employed with the force of gerund, and not every noun. The भाववाच्य nouns correspond to the nouns of action or verbal nouns.