Back to back expansions only after the minimum amount of time between expansions has passed and not otherwise limited by era allowances. What are you hinting at?
Talentz
The way I see it working is that by using up slots, pressure to expand airport will build up, and most the slots being used up creates the maximum pressure to expand.
Then, the expansion that also grants slots is like a pressure valve and releases the build up pressure, so that there is no pressure to expand until the slots are used up again.
But by not granting slots, there is no pressure valve, and there is equal pressure to expand before and right after the expansion.