This weekly Portion starts with the words: וְהָיָה עֵקֶב תִּשְׁמְעוּן אֵת הַמִּשְׁפָּטִים הָאֵלֶּה, “And it shall be as a consequence if you shall harken to these Mitzvot.” (Deut. 7,12) The Hebrew word for “as a consequence” is עֵקֶב. This word has another meaning in Hebrew. It means heel. Because the Torah uses this word Rashi presumes it is talking about those minor Mitzvot that one is apt to tread upon with his foot.
Besides meaning “minor Mitzvot” perhaps it also means performing a Mitzvah in a minor way. For example, instead of buying a nice set of Lulav and Etrog although he can afford it, one buys a cheap set to save money.
Another example would be if a poor individual asks you for a donation you give him a dollar when you know he could use more. That could also be considered trampling on the Mitzvah.
Rashi is trying to emphasize that all the Mitzvot have a value which we may not realize. Hence, it is important for us to do all the Mitzvot with our full heart. We never know what value they have or what effect they can have on others. Rashi implies the Torah refers to minor Mitzvot but it can also be referring to doing Mitzvot in a minor way.