I wanted to add an official answer of what the phenomenon is called. On the Today show, David Copperfield who served as a consultant for the movie actually explains what the reverse rain effect is based on. The trick is based on the Wagon-wheel effect or also referred to as the stroboscopic effect.
Essentially what happens is by playing around with lighting you can make it seem like something has a slower frame rate than it actually is. For rain, what you are seeing is the strobing lighting the rain in the correct intervals to make it seem like they are staying still, going up, etc.
David Copperfield reveals the secret to ‘Now You See Me 2’ water illusion
In the movie, if you go to the to around 45 minutes in, after the twins Merritt and Chase are talking, you'll see Wilder stumbles upon the device which is known as a strobe fountain that illustrates the trick (and I guess this scene was also to foreshadow the magic and give an explanation for the inspiration).