This game is available on some pub slots which are powered by Microgaming software. As a rule the pub slots are available at the online casinos using flash version.
So, the pub slots offers the WinSpins slot game on the Billion Dollar Gran pub slots. There is a regular slot game on the Billion Dollar Gran pub slots in which you bet and spin the reels. But winning combinations on the regular slot game give you not a coin, and some of free spins. As soon as you win the winning combination, the second screen with WinSpins slot game will be opened.
This game is started automatically on such amount of free spins which you have won in the basic game. The winning combinations on WinSpins slot game give you the coins. The bet in bonus game is that which you have made on the regular slots. At the end of spins your prize is added in casino bank and you are returned to the basic game.