How to slow down video on iPhone?

I've seen people post videos that start at full speed and slow down when they choose to and sometimes even go back to full speed after the focus of the video ends (a back flip for example). Anyone know how they accomplish this?

They use a video editor before posting the video.

Its called using a VIDEO editor. And in most cases the audio ends up kack during those periods because even a good video editor can't work out what to do with the audio.

IPhones have had the slo mo video option for a while now. Can't remember specifically which iPhone started it but at least ones from the iPhone 6 on up have it for sure.

Also could be edited after the video was shot using apps like iMovie.