It doesn't have to be multiple Pioneers. Just one will do most of the time.
1. Start building/training whatever you want the city to build/train.
2. Queue up as many Pioneers as you can behind it.
3. The turn before your improvement/unit finishes, cancel the Pioneers. Or let the thing finish and then cancel them if you don't mind the slight production loss (I think that's how it works) in exchange for the extra turn of growth.
Between steps 2 and 3, you got extra growth. Of course, there is a cap on the growth you can get from excess food. It is 3. So if you're already getting 3 growth from food, there's no point in doing this. The rest of the time, it's quite exploitable.
I'm just laying it out there for those who didn't think it through.