Is something that is given from the Freebitco.in website to people who use to gamble a lot and during such period the level of the rewards that you can get normally by wagering/gambling is multiplied.You normally during normal periods of gambling gets 1 RP and 1 Lottery tickets for every 500 Satoshi you spend and 1 Golden Ticket for any 500.000 Satoshi you spend.

During a flash bonus you have tiers like first tier gives you 100 lottery tickets and 100 rewards points and this x number of Golden ticket together with a number of FUN tokens as added extra if I recall correctly,then for every tier you pass these rewards are multiplied by x5 or x10 or even more for the final level,so in the end this bonus is a great incentive for people to wager during such period because of increased rewards.