The description for Magnetic Billiards Relentless game mode reads:
Balls from the tube will be gradually added to the table.
How long will you last?
However, I'm not entirely sure what actually triggers the addition of new balls. This is annoying because the bonus for completely clearing the table is very high, and I can't reliably do this without understanding when new balls will appear.
It seems to be quasi-random, usually waiting until there are only one or two single balls left not in clusters, but this isn't always the case, as you can see from the screenshot below.
What triggers this?