The shaking of the harness bells is an example of auditory imagery. Along with the sweeping sound of “easy wind and downy flake,” the tinkle of the bells was the only sound that could be heard in the woods.
This underscores the point that absolute silence gripped the woods on that wintry evening. It was, perhaps, the soothing silence that had caused the speaker to forget his promises momentarily.
Moreover, the poem can be read as an allegory to a man’s journey towards his goal. On his way, several impediments in the form of difficulties or enticements await him to lead him astray. It’s only when he’s able to overcome all such barriers, he will be able to reach his destination.
In the poem, the speaker had "promises to keep.” At no cost, he should stop midway through his journey. But, the mesmerizing beauty and the peaceful ambience of the woods seduced him to prolong his journey.
While he was lost in reveling in the comforting splendor of the woods, his horse shook the harness bells.
He gives his harness bells a shake
To ask if there is some mistake
Figuratively, the harness bell can be interpreted as the voice of one's conscience. One always knows when one does something wrong. It is one's inner voice that alarms one against the improper or wrong decision one is about to make or has already made.
Prolonging his stay in the woods would be tantamount to giving in to the sensual enticements offered by the place. As soon as the traveler began to gratify himself, his conscience warned him by asking him “if there is some mistake.”
So, we see that on the surface level, the auditory image of the ringing of the harness bells accentuates the sense of silence pervading the woods. At the allegorical level, it indicates the inner voice of conscience that is always right.