"Knowledge is power," I had always taken this phrase to be about the nature of power. As in, if you have more knowledge then you will be more powerful. In this construction the emphasis seemed to be on book learning and smarts. If you are more educated and read a lot then you will be able to press your agenda on others. If you are not educated then you are weak.
But a few years ago, in college I had a TA who appended the French phrase "Savoir c'est pouvoir," to an e-mail. Another student asked her in class what it meant and she replied that it meant, to know is to be able. This I took to be a statement about the nature of knowledge. That is, if you are able to do something then you know it. Or to take the inverse (assuming a biconditional), if you are not able to do something then you don't know it. This takes the emphasis away from book learning and puts it on practical experience and technical skill.
"To know is to be able," has a similar flavor to Pragmatist statements like "Meaning just is use," (Wittgenstein) and "the true is only the expedient in our way of thinking" (James). According to some (pretty unreliable) sources the pragmatists were inspired by the phrase "knowledge is power" (attributed to Francis Bacon). The proverb has the look of a Pragmatist theory of knowledge. As a sometime Pragmatist myself, I am attracted to this idea of knowledge as something which gets its value through practical use.
I hadn't realized that the two phrases were supposedly equivalent until today. The English phrase I had always disliked, because it implied that well educated people were the best able to effect change, while the French version I had liked because of the implication that if you couldn't actually do something then you didn't know it after all.