When you claim a 99% reward, your Progress Bar will immediately advance to the 99% mark. This ensures that you are only a few spins away from levelling up and receiving a Spinbox prize.
The 99% reward works best when you use it strategically. For example, if your Progress Bar is at 75% when you receive this reward, you will probably want to wait to use it until you level up through normal play and then claim the 99% reward when your Progress Bar resets to 0% on the next level. In this way, you can achieve two Level Ups within seconds and two free spins rewards via Spinbox.
But remember that like all rewards at Caxino, the 99% reward has an expiration of 24 hours. If the time is running out on this reward and your Progress Bar is mostly full, you should go ahead and use it, versus running the risk of the 99% reward expiring and gaining no benefit.
With this reward more than any other at Caxino, timing is the name of the game.