氵 is left-side water and grounds 涮 in the rinse/wash family — the classic image is meat dunked and swished in boiling broth (涮羊肉, hotpot mutton). Water and motion together define the cooking action.
刷 gives the sound shuā → shuàn (rhyme change and tone shift). There's a faint semantic echo too — 刷 is "brush, scrub" and 涮 is "rinse, swish" — both rapid back-and-forth cleaning motions. Mostly phonetic though.
