“Casting them beneath their heels” means that a person agrees that these mitzvos must be performed, but only after the more serious mitzvos, and without the same degree of care devoted to them.
Regarding the verse, “And it shall be, because you will listen,” the Midrash explains: “The lighter mitzvos which people cast beneath their heels.” The Midrash continues that this is what David meant when he said, “Why should I fear in days of evil? The sin of my heels surrounds me.” He did not fear the serious mitzvos, but rather the lighter mitzvos.
The explanation is as follows:
The intention is not that a person entirely fails to fulfill the mitzvah. Were that the meaning, the emphasis should have been only on the words “which they cast away,” and the additional words “beneath their heels” would have no significance.
Rather, a person agrees that these mitzvos must be performed, but he places them after the more serious mitzvos and does not observe them with the same degree of care.
He argues that there is an order established by Torah: first, one must fulfill all of one’s actual obligations, and only afterward may one begin to consider additional levels of meticulous observance, conduct beyond the strict requirement, Chitas, and similar matters.
This, however, is the counsel of the yetzer hara. The proper order for accepting and fulfilling the Torah is “We will do,” and only afterward, “we will hear”—an acceptance that transcends the limitations and orderly calculations of human reason.
Through this, we merit the fulfillment of the promise, “Hashem your G‑d will preserve for you the covenant,” in a manner that transcends all measure and limitation.