In an 1829 text by August Bournonville, he defines Epaulement as follows:
"Epaulement: Opposition
The straight lines or those that show the whole body quite en-face are probably most useful in study, but they have for good reason been modified for the stage in the newer school. The second position is for example maintained by the stretching and lifting of the leg (I assume the left one). It becomes more beautiful by a slight turn or Epaulement to the left side, this is called effacé. In the fourth position the leg is placed straight forward; with a slight turn of the shoulder to the right, it is called croisé.
It is known that arms and shoulders function in opposite movements to the legs: one could only walk with difficulty if one placed the same foot, arm and shoulder forward simultaneously when stepping out. This rule of nature is a dogma in the dance, in that whichever croisé position the left foot is placed when forward, the head must turn to that side, the left shoulder and arm must be lowered and keep more forward than the right, which when raised, so to speak, frame the upper half of the body. However, in effacé position the head must turn towards the raised arm. Any position which broke with this rule was condemned by the older masters and called a false opposition, but Vestris, the creator of the new school, discovered partly by studying antiquity, party by his own inspiration, that one could swerve from the strict rules, remaining graceful, yet not foreshortening the older, beautiful oppositions. He thus enriched the dance with innumerable others, and proved thereby that head and shoulders in different positions allow the arms fortuitous curves, which although not in opposition to the legs, far from hampering these movements, increased their grace."
While it is more commonly accepted today to think that "epaulement" means only how we move our shoulders, thus moving/twisting our upper body/back, epaulement historically is closer to the way russians use it (as usually, they are more traditional), meaning the whole body positioning and how it is perceived by the audience.