This happens when the droplet launching up from the spout at it's height is still going up so that when a second droplet coming down hits it then it punches through the middle.
Canon Speedlite 580EX-II with light blue gel firing from about 30cm up down at 45 degrees. 430 EX-II below it firing directly into the back of the splash (Darker blue gel) with a frosted perspex diffuser about 30cm in front of them.
Close-up and macro
Derek_Fearn, ali63, senn and 55 more