From out Terms and Conditions:

From time to time, as determined in our sole and absolute discretion, we may provide you bonus funds on deposits to your Account or purchases of virtual currency  (“Bonus Cash”). Bonus Cash can be used to enter Games, but cannot be withdrawn or used for any other Services. When you enter a Game, your entire allotment of Bonus Cash will be spent on the Games entry fee. If Bonus Cash does not cover the Games entry fee amount, then the remainder will be covered by the funds maintained in your Account. When you win a Game, any Bonus Funds that you have used to pay the entry fee will be returned to you and any additional winnings beyond your entry fee will be paid in U.S. Dollars. Bonus Cash shall be treated similarly to Virtual Items throughout the Website.