According to Ringeck, Liechtenauer is specifically talking about the first cut in the engagement. The concern is that if a right hander cuts from the left he may be find himself in a bind in the left, which is disadvantagous to him.
Likewise left-handers should open with a cut from the left.
Tobler adds that the initial blow will also be weaker. He continues by saying the bind is weaker because the wrists must cross and that makes the grip tenuous and disarms possible.
Timothy J Owens: The thought about this during practice, as a left handed fencer when I make a strike from right vomtag I feel at a disadvantage against a right handed fencer. Similarly a strike from left vom tag feelsmuch stronger and more natural. A modernexample would be to swing at a baseball from your dominant side and then tryfrom the opposite side.