Most likely it was created by the way the bucket was moved during the throw, not by any special darkroom trick. If the person tossing the water launches it forward, then quickly drops or pulls the bucket back, the water already in motion keeps traveling while later water leaves at a different angle and speed. That can produce a splash with multiple apparent curves rather than one simple arc.

In other words, the water’s shape depends not just on gravity, but also on the changing motion of the container at the moment the water leaves it. If the source moves rapidly compared with the water’s exit speed, you can get surprisingly complex forms.

So the effect is mainly timing, bucket motion, and repeated attempts. The exact shape probably wasn’t perfectly planned; it was likely one of many throws, with the goal also being to keep the thrower’s arms out of the frame. A fast shutter or flash would then freeze the splash at the right instant.