The phrase "It's a ding ding" is used when experiencing a big win in Casino games, a win 100x or more is normally required for it to be a Ding Ding. Higher wins 300x or more are often referred to as a SUPER MEGA DING DING.
The phrase is said whilst holding one foot with both hands, then raising it in the air as you say "ding ding"
Legend has it that a Swedish man called Sir Kim from Halmstad created the phrase when getting over 100x on Starburst!