I have never heard that expression, but here is what happens after you get implants under the muscle. Initially they feel hard and tight. It takes weeks for the muscle and skin to loosen up and stretch out to the point the implants feel softer. I suspect that is what you are talking about.
Breast implants do not change size after surgery but the position of the breast implant on the chest wall and in relationship to the breast tissue changes over the first 3 months. Generally speaking the breast implants, especially under the muscle, are spread out and high on the chest. As the breast implants drop or moved down into the space beneath your breasts the breast shape appears more natural and Fuller. Some plastic surgeons, but not me, referred to this change in position as "fluffing". The breast changes after breast augmentation are typically that the breast appears more natural but not truly larger.
What does it mean to wait for your implants to "fluff"?
I have absolutely no idea what you are referring to with that term and no, breast implants never change size unless they leak.
In all my years in plastic surgery I have not heard this term applied to breast implants.
Volume is determined by dimensional analysis of the breasts. Understanding your goals and the breast aesthetic you desire.
There is no great practical method to demonstrate this. There are 3 dimensional imaging devices which can predict how you might look after the surgery but your best bet remains clear communication with a surgeon who understands your aesthetic and will deliver it.
I hope this was helpful.
Fluffing After Breast Augmentation
"Fluffing" refers to the impression that the lower breast enlarges and that your skin softens with the creation of new tissues. The extent to which this occurs depends on several factors, including the implant size, the tightness of your skin, and your body shape.
Therefore, your breasts will appear more natural. The volume will remain the same, but it will just be redistributed.