This is a pretty clear experiment to try out a new source of cash flow for Twitter, which saw a drop in advertising revenue this year, making $439 million in Q2 2017 compared to $535 million in the same period last year. Twitter confirmed to Engadget that it's currently testing this program, but didn't comment further.

Interesting @Twitter ads are running a new private beta program https://t.co/RbIEHMnGNl pic.twitter.com/oStFltf47Q — David Iwanow (@davidiwanow) July 28, 2017

Users that had paid Twitter to promote their tweets before were invited to the pilot program, though interested users can apply for the program now and get 30 free days of "automated promotion." According to the FAQ section, it will only apply to the first ten tweets of the day, and they will go out to either users with similar interests or those in a chosen metropolitan area (but not both).