What does the cryptic crossword clue "I don't know what my self-flagellation does (2,5,2)" mean?

The definition is "I don't know". The wordplay uses a double definition. Double definition The answer is ITBEATSME.

I don't know what my self-flagellation does (2,5,2)

Guardian Cryptic #28134 | 6 Down | 2020-05-15 flower