Well, By using is part of an adverbial phrase, whereas Using, on its own like that, is a noun, and because it ends in -ing, it's called a gerund. In the example sentences below, "Using" is a noun, and nouns functions as objects and subjects, so in 1. the noun "Using" functions as the subject of the sentence. In 2. the phrase "By using" functions as an adverb, specifically an adverb of means; it answers the question How?, and since adverbs can move around the sentence, "By using, can be moved to the end of the sentence, just like in 3.:Originally Posted by RedWood
1. Using a toothbrush can prevent cavities.
2. By using a toothbrush, you can prevent cavities.
3. You can prevent cavities by using a toothbrush.