من ذاق شيئا بفمه لم يفسد صومه
Simply tasting something in the mouth does not invalidate the fast.
The argument distinguishes merely detecting a taste in the mouth from swallowing a substance. One may notice the taste of water when rinsing the mouth without having eaten or drunk in the legally relevant sense.
This does not recommend unnecessary tasting, nor does it imply that swallowing food or water is harmless to the fast. The limited issue is whether taste sensation by itself invalidates the fast.
Accurate attention to the verb “to taste” prevents confusing sensory perception with consumption.