The affix kyaṅ is used in the sense of ejecting after the words vāṣpa 'vapour' and ūṣma 'heat' as the object of the action.,
Thus, बाष्पायते 'it emits vapour'; उष्मायते 'it sends out heat'. So also फेनायते 'it ejects froth'.,
बाष्प-ऊष्मभ्याम् उद्वमने
Adhyāya 3 · Pāda 1 · Rule 16