After that verb that has an indicatory ṭu , the affix athuc comes with the force of denoting mere action etc.,
As टुवेपृ--वेपथुः 'a trembling'; टुओश्वि--श्वयथुः 'swelling'; टुक्षुः--क्षवथुः 'sneezing'; टुदु--दवथुः 'burning'.,
ट्वितो ऽथुच्
Adhyāya 3 · Pāda 3 · Rule 89