There are at least two main schools of thought on this, going back a ways. In the RWS deck, the Woman is trying to force the jaws of the Lion closed, suggesting mastery of the animal nature through an exercise of will. In the TdM decks - and in the B.O.T.A tarot - you can see that the Woman is trying to open the Lion's jaws, giving expression to the more instinctual side. I personally like the second version because it implies creativity, while the former looks more like suppression. Although Leo is a fixed sign, I see it as more joyful than "buttoned-down." Modern versions kind of duck the question because they often show a Woman posed benignly with a Lion, implying that she has already reached peace with her primitive urges. How she reached that point isn't implied in most cases.
One of my favorites among the "peaceful" Strength cards is in the Anna K, and it was the main reason I finally bought the deck.