Terry, I'm sure others could describe it more eloquently, but I'll give it a shot.
When you are awarded the 'OK' feature on Bikini or Lido, you do not have to play the red letter game right away. In fact, instead of highlighting a letter on the backglass, a number is lit, indicating the number of futurity increase with two or more in the orange section.
Playing a futurity game is similar to a red letter game. Pressing the orange button on the footrail will play off the futurity game banked. There is a separate instruction card that indicates which odds and features are awarded for each futurity game number.
When the orange button is pressed, the current game immediately ends (or a new game starts), all the balls drop, and the odds and features rack up based on the earned futurity number.
What makes Bikini (and Lido) unique is that you can bank them. Earn a two increase in futurity? Rather than having to play a low level red letter game, keep playing until you earn another low level. Then you have a much better red letter (OK) game ready to play.
I'm using 'red letter' to describe the actual guaranteed free game. The futurity button provides this.
Don't know if that's more confusing? I'll re-read it later and see.