Hi, Mahmoud Jaber,
Thanks for your reply, Gustavo. However, it's not yet clear enough what you mean. Can you clarify more?
What Gustavo means is well explained by Rachel, our late moderator, in the link below:
"We don't usually say something like "he is sinking," because it doesn't happen very often.
A true story: A year-old baby I knew was playing at the edge of the swimming pool. Before anyone could catch him, he fell into the pool and sank to the bottom. His mother saw the fall, of course, and jumped in after him and picked him up and out of the pool. He was smiling!
The baby had sunk to the bottom of the pool, and his mother had saved him from drowning."
So, your example simply means that a lifebelt is thrown to a person who has fallen into water to stop them from sinking / going down to the bottom.