The player downloads the next stretch of video before you reach it. When the connection dips it plays from that reserve, and stalls only when it runs dry.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
filling the bucket before you pour the glass.
For example
The movie pauses and a little circle spins. It is not stuck — it is quietly collecting the next part before carrying on.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.