Ah okay. So that button is to dominant Open the movie and check for autostart." is just the preperation to check if the video starts correctly e.g. if in firefox autoplay is allowed.
The actually watch party starts when the countdown hits zero, by redirecting everyone in the same moment to the video.
"Open the movie directly and ask in the voice chat what time you should jump to" is when you missed the start.
Syncing works via starting everyone at the same time in the beginning of the movie
If for some reason somebody joins late / gets kicked off, and you're watching something on youtube, youtube supports time-index URL fragments like this https://www.youtube.com/watch?v=F4-SxcCO5d0#t=3m17s , and you could start a new watchparty with that URL.
It's heavyweight and probably better to just sync by somebody shouting a timestamp, but if you need things synced there you go.
Just calculating from the start time at what point in the movie everyone should be. e.g. You are half an hour late, so the others are around minute 30 (Y)
The actually watch party starts when the countdown hits zero, by redirecting everyone in the same moment to the video.
"Open the movie directly and ask in the voice chat what time you should jump to" is when you missed the start.
Syncing works via starting everyone at the same time in the beginning of the movie