This seriously amazes me. Science is a great magician in many ways.
This is really simple but has such an awesome effect. Fill a bucket full of water and place it about 5 feet off the ground. Place a subwoofer about 1 foot lower than the bucket. Run a plastic tube from the top bucket down in front of the subwoofer. Tape the tube to the front of the speaker. Then aim the end of the tube to an empty bucket on the floor. Get the water flowing from the top bucket. Now just generate a 24 hz sine wave and set your camera to 24 fps and watch the magic happen. Basically your cameras frame rate is synced up with the rate of the vibrations of the water so it appears to be frozen or still. Now if you play a 23 hz sine wave your frame rate will be off just a little compared to the sine wave causing the water to “move backward” or so as it appears. You can play a 25 hz sine wave and cause the water to move slowly foward.