The balloon rules
You start with three balloons. A pass or a timeout removes one. A 100-point answer restores one, up to the maximum of three. The run ends when you press Next with no balloons left.
An answer that is not on the board does not remove a balloon. It only costs time, because the round continues until you answer correctly or the fifteen seconds expire.
What a balloon is worth
A balloon is a second chance at a full round. The value of protecting one is therefore higher than the value of a single answer, because every additional prompt can add up to 100 points. Trading a balloon for a speculative answer is usually a bad exchange.
The exception is the last balloon. With one balloon left, a timeout ends the run anyway, so the cost of a rejected guess is only the remaining seconds, while the potential gain is a full new round if you find the 100-point answer.
When to pass
Pass when you have no candidate and the timer is nearly gone. Do not pass to skip a prompt you find difficult: a difficult prompt still has twelve answers, and some of them are ordinary words you can find with a few seconds of thought.
In practice, most passes in Endless happen too early. A rejected guess is free in balloon terms, so the correct default is to keep submitting candidates until the clock forces a decision.
The right risk profile
In Classic, a single weak prompt costs at most 100 points. In Endless, a timeout can end the run. That makes consistency more valuable than a spectacular answer. Choose the plain answer more often than you would in Classic.
When you have three balloons, you can afford one aggressive attempt. When you have one, play the safest answer you can name.
Comparing Endless runs
Endless has no seven-prompt ceiling, so its total is not comparable to a Classic score out of 700. Compare Endless runs with other Endless runs, and look at prompts survived as well as points earned.
Recovering after a lost balloon
After a lost balloon, the temptation is to chase a 100-point answer to restore it. That is backwards. The 100-point answer is the plainest answer, not a special one, so chasing it produces the same behaviour as playing safely.
Treat the next prompt as a normal round: read the qualifier, choose the obvious answer, and take the 100 if it comes. A deliberate attempt to be clever is the most common way a one-balloon run ends immediately.