Hang around a large-enough Twitch channel and you'll eventually see a sub or bit train -- a moment where the hype kicks off and the streamer ends up with a mountain of donations. Twitch wants to seize on those sprees. It's launching a Hype Train feature this week that hands out free emotes to donators when they fill up an on-screen meter. A train starts when enough subscriptions or bit cheers occur in a certain amount of time (streamers can choose a threshold). After that, any subs or 100-plus bit cheers contribute to the meter. There are five levels of emotes with five levels each, so you'll have an incentive to keep frenzies going for a while.
The feature has been in beta for weeks, but should be widely available to both affiliates and partners. They're enabled by default, but can be turned off.
There's no mystery as to why Twitch would do this. Sub and bit trains contribute directly to the service's bottom line -- it wants to encourage that behavior as much as possible. At the same time, it's hard to see many users objecting to Twitch's move. These frenzies already happen organically, and you might as well get something for your effort besides the satisfaction of supporting a worthy broadcaster or a charity.