Here's an example. In the system we had last week, the event would start at (for example) 10:00. As soon as the first person joined the instance, the countdown timer would begin. More people are still joining, because it's the appointed time and people were waiting in EotN. But, sometimes people take a while to load in and the timer might be near 0 by the time some people get in, giving them little or no time to prepare.
In the new system, people would still hang out in EotN leading up to 10:00, and would still enter the public instance as they did before, but the timer wouldn't start until "enough" people were in - at that point, the countdown timer starts giving everyone time to prepare.
If you're late getting to the Scrying Pool and load into an instance without enough people, you could (a) wait around hoping enough people show up, or (b) exit and wait for 12:00. In the old system, the timer would still start, and you'd have no choice but to go with (b), or (c) start anyway and fail.