Flexi betting is a feature offered by some bookmakers that allows you to increase the number of selections in an exotic bet, without increasing your outlay.
Below is an example of Flexi Betting with a Quaddie bet:
First Leg: #2 Prince of Sussex & #4 Moonlover
Second Leg: #8 Mr Quickie & #14 Frankel
Third Leg: #3 Widgee Turf
Fourth Leg: #4 Ashlor & #3 Iconoclasm & #10 Miss Leonidas
There are 12 combinations in this bet, and if you wanted to win 100% of the dividend in the prize pool you'd need bet $1.00 per combo, so the stake for this bet would have to be at least $12.00.
If you didn't want to invest this much, you could select the Flexi Bet option with your bookie, so that you can invest less and get a proportional amount of the dividend if successful.