Please look at Sotah 21a, specifically these lines:
- ויש זכות תולה ג' שנים כו': זכות דמאי אילימא זכות דתורה הא אינה מצווה ועושה היא אלא זכות דמצוה
רבינא אמר לעולם זכות תורה ודקאמרת אינה מצווה ועושה נהי דפקודי לא מפקדא באגרא דמקרין ומתניין בנייהו ונטרן להו לגברייהו עד דאתו מבי מדרשא מי לא פלגאן בהדייהו
(and the English)
- AND ANOTHER FOR THREE YEARS etc. What sort of merit? If I answer merit of [studying] Torah, she is [in the category] of one who is not commanded and fulfils!6 — Rather must it be merit of [performing] a commandment
Rabina said: It is certainly merit of [the study of] Torah [which causes the water to suspend its effect]; and when you argue that she is in the category of one who is not commanded and fulfils, [it can be answered] granted that women are not so commanded, still when they have their sons taught Scripture and Mishnah and wait for their husbands until they return from the Schools,15 should they not share [the merit] with them?
The Gemara comments on the idea we have just seen in Sotah. What kind of merit does a woman have to stave off the water (to prevent her from dying?)
If you say she learned Torah, well, she's not commanded or obligated in learning Torah! So it must be that the merit comes from the other mitzvot that she is obligated in performing.
Ravina says- "You'll say she's not commanded to learn Torah- granted that she's not obligated to learn, but still the fact that she works for the education of her children- in that merit, and also because she waits for her husband till he comes home from learning- so for that she splits the reward with the man!"
Look at Rashi here, which clarifies that it's not that the woman is herself busy with Torah, but rather that she makes her son and husband busy with Torah, and moreover, that she brings her children to the school so that they may learn to read the Torah and the Mishna.