You put your pillow in and is like placing it in a balloon. It takes a bit to deflate when you place your head in it, but then as you move your head the air gets trapped and takes a while to get out. and because it does not breath well, it can turn any pillow into a warm pillow. It totally gives whatever pillow you have and spend money on a brand new feeling which is not comfortable.
When I received these, I was reluctant to try them. I felt the fabric and thought - my head's going to sweat, and they're going to be noisy every time I turn. After the first night, my results were - wrong and wrong! I've been using them for several weeks now and really like them. They keep the pillows nice and clean, too, and wash beautifully.
I have an expensive contour pillow and needed a cover to protect it. The cover fits very well on that and my standard pillow. I am always hot and was afraid that this cover would make my head hot at night, but it doesn't at all. Very pleased with this purchase.