Praise be to Allah.
If water reaches the throat, it spoils the fast, unless that happened without one intending it to, such as when rinsing the mouth or nose and the water reaches the throat. In that case, it does not spoil the fast, even if the individual went too far in rinsing the mouth or nose.
It says in Sharh Muntaha al-Iraadaat (1/483):… Or he rinses his mouth or rinses his nose, and water enters his throat without him intending it to, or he swallows what is left of droplets of water after rinsing his mouth, that does not spoil the fast, even if he rinsed his mouth or nose more than three times, or went too far in doing it, or it was because of dirt – that does not spoil the fast, because of the hadith of ‘Umar, when he asked the Prophet (blessings and peace of Allah be upon him) about the fasting person kissing, and he said: “What do you think, if you rinse your mouth from a vessel when you are fasting?” I said: There is nothing wrong with it. He said: “Then there is no problem!”
And because it reached his throat without him intending it to, like dust.
It is makrooh to rinse the mouth or nose for no reason, or to go too far in doing it, or because of heat, or because of thirst. That was stated by Imam Ahmad, who said: Sprinkling water on his chest is better, in my view, such as if the fasting person immerses himself in water, it is makrooh if it is not for the purpose of ghusl, as required according to Islamic teachings, or to cool himself off. According to other scholars, these two actions are not makrooh. And it is more appropriate according to the Sunnah for one who is junub to do ghusl before Fajr.
If a person immerses himself in water and it enters his throat, that does not spoil his fast, because he did not do that deliberately. End quote.
The uvula (the piece of flesh that hangs down at the back of the mouth) is not part of the throat; rather it is hanging over it.
It says in al-Misbaah al-Muneer (2/559): The uvula is the piece of flesh that hangs above the throat, in the furthest point of the mouth. End quote.
Some fuqaha’ defined the outer part of the mouth as starting from the point of articulation of the Arabic letters khaa’ (خ) and haa’ (ح).
With regard to the inner part of the mouth, it starts at the point of articulation of the Arabic letters hamzah (ء) and haa’ (ه), even though the scholars of Arabic language state that the points of articulation of all these letters are in the throat.
It says in Nihaayat al-Muhtaaj (3/165): What is regarded as the end of the outer part of the mouth is the point of articulation of the letters khaa’ (خ) and haa’ (ح), according to the author [an-Nawawi]. As for the inner part of the mouth, it ends at the point of articulation of the letters hamzah (ء) and haa’ (ه). The meaning of throat according to the fuqaha’ is more specific than the meaning according to the scholars of Arabic language, because according to them the letters khaa’ (خ) and haa’ (ح) are “throat letters”, and the articulation point of khaa’ (خ) is lower than the articulation point of haa’ (ح). Then there is the inside of the mouth and nose up to the end of the epiglottis. End quote.
The epiglottis is the top of the throat.
Al-Misbaah al-Muneer (2/450).
See: al-Majmoo‘ (6/319). It is more likely that the articulation point of the letter haa’ (ح) is in the outer part of the mouth, unlike the view of al-Ghazaali and ar-Raafi‘i, and the uvula is part of the outer part of the mouth; it comes before all the articulation points of the throat letters, whether the articulation point of the letter haa’ (ح) is said to be in the outer or inner part of the mouth.
With regard to gargling, a concession allowing it is granted in cases of need, on condition that nothing is swallowed, otherwise it reaches the throat and the inner part of the mouth.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Gargling [when fasting] is makrooh, except in cases of necessity, because the Prophet (blessings and peace of Allah be upon him) said to Laqeet ibn Sabrah (may Allah be pleased with him): “Rinse your nose thoroughly, unless you are fasting.”
If a person needs to gargle, and he cannot delay it until he breaks his fast, then there is nothing wrong with him doing that, but he must be extremely careful not to let anything go down into his stomach.
End quote from Majmoo‘ al-Fataawa (19/255).
In another answer that he gave concerning gargling for one who is fasting, he said:
It does not invalidate the fast if he does not swallow anything, but he should not do it unless it is necessary. It does not break your fast if nothing enters your stomach.
End quote from Majmoo‘ Fataawa ash-Shaykh Ibn ‘Uthaymeen (19/190).
And Allah knows best.