Well Twitter comes in 2 parts in my opinion; Part 1 would be your "promotion" YouTube has an automatic posting to Twitter and G+ when you're uploading a video you can type your msg and bam, it'll be on twitter once you connect your account, which is nice an all. It's how I get my videos onto Twitter without thinking too much about it.
Will that blow your channel up? Nope.
If all you post is promotional videos you won't get anywhere really, so... Part 2 "contribution" basically same concept as posting here on the forum, you have to contribute to Twitter, interact with other tubers, posters etc, reply to their posts, retweet things, like them and so on and so forth. Get people to know YOU and as people start liking your content on Twitter (IE your contributions to random things) They'll probably follow you on Twitter, and then they'll see your automatic YT uploads, and since they already know a bit about your personality and style they will probably check it the video out. And then if they like the video they might sub.
It's a process really. It can pay off, but it won't fall into your lap overnight.