How does Rashi learn from the dots over the word את that the brothers went to pasture themselves?
(37,12) “And his brothers went to pasture their father’s flock (לרעות את צאן אביהם) in Shechem.”
Rashi explains that the dots over the letter את (there are several words in the Torah which are written with dots over some, or all, of the letters) are to teach us that they went to pasture themselves. That is, although obstensively they went to pasture their father’s flock, the Torah hints to us that their main intention was to feed themselves.
The logic behind this teaching is that although some words are dotted to teach us that there is hidden meaning, nevertheless, the hidden meaning must be related to the word which is written. Therefore, in this posuk the hidden meaning is derived from the fact that the root of the word את is אות, which forms the words אותו which means him or it and אותם which means them.
But these words can also be understood to be reflexive, and thus mean himself or itself and themselves. An example of this is the posuk in Bamidbar 6:13 “This is the law of the nazir: on the day that the days of his being a nazir are completed יביא אתו to the entrance of the Tent of Meeting”, where the phrase יביא אתו means "he shall bring himself", as Rashi explains. Rashu adds that this is one of the three places in the Torah where R. Yishmael teaches that the word את and its suffix are understood to be reflexive, the other two places being in Vayikra 22:16 “and so cause themselves (אותם) to bear iniquity and guilt”, and in Devorim 34:6 “and he (Moshe) buried himself (אתו) in the valley”.
Hence, Chazal deduced that the hidden meaning of this posuk is that they went to pasture themselves.