Or HaChaim on Deuteronomy 11:27/אור החיים על דברים י״א:כ״ז

1 את הברכה אשר תשמעון, "The blessing if you will hearken, etc." The word את which frequently means "with" means here that in addition to the blessing which is the result of walking in G'd's paths there would be a bonus, i.e. they would hearken to G'd's commandments. Hearkening to G'd's commandments is perceived as a pleasurable experience by itself. It helps the soul to feel "alive" as we know from Isaiah 55,4: שמעו ותהי נפשכם, "hearken and your souls will come alive." Whenever someone who studies Torah gains an understanding of what the Torah has in mind he experiences a physical and spiritual sense of wellbeing. He owes G'd a debt of gratitude for affording him such pleasure. There is no need to add that such a person cannot demand a reward from G'd for having allowed him to experience such joy.

2There is yet another aspect to the words את הברכה. We have been taught in Kidushin 40 that the reward for מצוה-performance is divided into "capital and dividends." When Moses uses the word את he refers to the "dividend" aspect of the blessing which is the reward. The word הברכה refers to the world in which the capital is payable, whereas the word את refers to our world in which the dividends of this capital are paid out.

3Moses was careful to say אשר. Had he used the customary word אם this would have had a connotation of "if," i.e. that the payment of such dividends was somehow subject to doubt. By saying instead אשר, Moses made sure that this dividend would be perceived as something absolute, not subject to doubt or additional conditions. The Torah made this whole blessing dependent on "listening" rather than on performance of a deed or deeds with one's limbs as it wanted to convey the idea that anyone who listens to the living G'd and His Torah becomes imbued with the elixir of life, something which is guaranteed to bring him back to the fold in case he had strayed (compare introduction of Eychah Rabbati).