This is set to roll out "shortly," according to the FAQ, and the press release points towards Spring of 2017. Here's the details video provided by Twitch that gives the service a quick rundown:

How will streamers earn revenue?

Twitch pitches this to gamers and viewers as a way to support streamers. That's entirely true. Twitch actually provided the breakdown of where each purchase goes.

If you buy a game on the stream, 5% of the sale goes to the streamer, 70% of the sale goes to the developer and the rest, we assume, goes to Twitch. Here's what'll look like on an actual streamer's channel, courtesy of Twitch.

I'll just float this notion out there as new media continues to evolve. Clearly, there are tons of reasons to dig into a stream. You might like the streamer, you might be interested in the game and you might actually be looking to make an informed purchase. This move, to me, turns streamers into advertisers. That was always sort of implied, but this just makes it overtly apparent.

Which is interesting. Will viewers reconsider a purchase if the person demoing the game for them gets a cut? Will streamers move towards only playing games they can sell? We'll see.