War and Peace is chock full of repetitions; who can forget Princess Marya's "luminous" eyes? In this passage, Tolstoy draws out attention to a tic of Napoleon's -- a rhythmic trembling of his left calf. Balashov is with Napoloeon and has been tasked to tell Napoleon the following -- "that not a single armed enemy remains on Russian soil". This is his directive from the tsar. Yet he is unable to say this directly to Napoleon. All that Balashov asks for is that Napoleon withdraw his troops beyond the Niemen river.
On the next page Napoleon comments that the trembling of his left calf is a great sign to him. A sign of what is unclear. He does not heed Balashov's polite request and continues his march on Moscow. -- Lola
from page 25-26 of original text, Volume 2
page 619-620 Pevear/Volokhonsky translation