It takes your slides (PPTX format), and analyses each one a bunch of ways, to spot common pitfalls. For example, it checks the reading age, how busy the slides are, how colourful (or not) your colour palette is. It also checks if you put too much text on each slide.

It's not fool proof, but is surprisingly accurate for various slide-fails :D

Here are some formulas for calculating readability for a text: https://en.wikipedia.org/wiki/Readability#The_popular_readab...

Neat - we roughly implement the Fog algorithm