One of the sections of Moshe’s farewell speech opens with a reiteration on the importance of energizing actions with effort:
וְהָיָה עֵקֶב תִּשְׁמְעוּן, אֵת הַמִּשְׁפָּטִים הָאֵלֶּה, וּשְׁמַרְתֶּם וַעֲשִׂיתֶם, אֹתָם-וְשָׁמַר ה אֱלֹהֶיךָ לְךָ, אֶת-הַבְּרִית וְאֶת-הַחֶסֶד, אֲשֶׁר נִשְׁבַּע, לַאֲבֹתֶיךָ – When you will finally listen to the laws, observe and perform them; Hashem will safeguard you, and uphold the covenant sworn to your fathers. (7:12)
The c conditional protection raises an issue. We are not supposed to observe our duties as workers expecting compensation; we are meant to dedicate ourselves because it is objectively important.
So why is observance framed with the conditional incentive of protection?
The Alshich notes that the word עֵקֶב is very peculiar, and not frequently found. It also doesn’t seem to add anything to the message.
R’ Shlomo Ganzfried explains that the reward is not the outcome of observance itself; it is for the effort and exertion the word עֵקֶב implies.
The Gemara in Berachos tells of how R’ Zeira took a short break from his learning and left the study hall. He sat on the steps outside so that if a scholar walked by, he could stand up out of respect, gaining merit while being idle from his learning.
In other words, beyond any particular of set mitzvos and laws, his attitude was an independently valuable characteristic to display and exercise.
The Torah always requires witnesses to testify, without getting paid. However, they can be still be paid for their time or travel, because the payment is for the work and effort put in, and not the testimony.
The same is consistently true of the Torah’s affirmation of rewards. There may not be a reward for the actual mitzva in this world – but there might be tangible benefits to showing respect for the things the Torah considers important.
This might be why the word עֵקֶב – literally “heel” – appears here. Our legwork will safeguard us because effort makes all the difference.