I clicked various times to the right, didn't make much sense.
I came back, started clicking it down, now it made sense, until couldn't, so I clicked right.
Then it clicked, took me something like ten seconds to figure it out, and I am not known to be quickest knife in the shed.
Yeah, I dislike comments on format but on mobile the site was just unusable for me. I couldn't figure out which slide was the "correct one" to move forward, and even the zooming gesture would move me to another slide. I think the video presentation would be a better link than the slides.