This requires a repetitive motion and good lighting. You expose the position of the droplet on successive oscillation, but capture it at a slightly different spot on each.
Thinking about it from the perspective of a ball bouncing. On the first photo, I take a quick photo of the ball on the ground. The ball bounces up and then back down. I then take a photo on the second bounce of the ball just a bit off the ground. The ball goes up and down again. I then take a photo on the third bounce of the ball just a little further off the ground. That's why it is key that the oscillation of the object and the framerate be so close. Each photo is only .06 hz worth of movement away from the previous at the time the photo is actually taken.
When I finish, I end up with a slideshow that shows a ball bouncing very slowly, but in reality, it was well timed photos of many bounces.
This is referred to as the wagon wheel effect because the gaps in the wheels could make it so that things would seem to move slower or even backwards because of the interruptions in vision and where things were positioned when vision was restored. Our brains then would fill in the in-between bits in the wrong direction or assume it was the shortest distance instead of seeing what actually happened.