If water reaches the stomach without the fasting person exceeding normal rinsing or being excessive, their fast remains valid.

However, if they exceed the normal practice or exaggerate in rinsing, causing water to enter the stomach, their fast is invalid.

This is because excessiveness in rinsing is prohibited for a fasting person, as the Prophetﷺ said: "Exaggerate inhaling water during ablution, except when you are fasting." [Narrated by the Four Imams]

Transgression (Ta‘addi) refers to rinsing the mouth more than three times, while exaggeration (Mubalaghah) includes gargling, drawing water deep into the nasal passages, or filling the mouth with water in an unusual manner.