You can't clear a full-face snorkel mask like a traditional pair of dive goggles. The design of this kind of mask lets you breathe through your mouth or nose and you cannot create pressure inside the snorkel mask to clear it. The air you exhale will go out through the snorkel. You can clear a traditional mask by leaning back and exhaling through your nose. This will force the water out of the mask. It does not work like that with full face designs. You will have to get your head out of the water which will then allow the purge valve to open and let the water drain. Overall, it's not a big concern but if you are used to a traditional snorkel mask and the way to clear it, you will have to get used to not being able to do that.