Transitive verb and Ichidan verb
しずめる
彼はソファに身を沈めながら、アカデミー賞の授賞式の生中継を観ています。
He is watching the broadcast of the Academy Awards ceremony and sinking into the sofa.
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means sink so the verb vocab version is to sink. This is when you're doing the sinking.
When you sink something, like a battleship, you go on and say: "she's (しず) sunk, I sunk her." That's like your tagline, or something. You say it for some reason because you think it's cool.