When this mode is turned on you will see a progress meter and play/pause control in the bottom left corner of your presentation. This is useful so that you can assume manual control when needed. Here's an example of how it looks in action http://slid.es/news/autoplay.

If you want a completely clean presentation without any overlaid user interface elements you can append special variables to the query string. Here's an example which will make the presentation autoplay every five seconds, as well as hide the controls and turn on looping: https://slides.com/news/make-better-presentations/fullscreen?autoSlide=5000&autoSlideStoppable=false&loop=true&controls=false&progress=false

Auto-refresh
If your presentation will run in a kiosk/information display setting you can make the presentation reload automatically to ensure that any edits you make are visible. This can be enabled using a query string parameter called "autoRefresh" which accepts a number in milliseconds (1/1000 of a second).
For example the following will reload once every ten minutes: https://slides.com/news/make-better-presentations/fullscreen?autoRefresh=600000 (10m*60s*1000ms = 600 000ms)