That's probably one of the most important tips you could get right there. The reason being, if you suffer a Shallow Water Blackout (SWB - You'll hear this term A LOT from freedivers), the snorkel left in your mouth leaves an open passage for water to enter directly into your lungs. Taking the snorkel out of your mouth increases your chances of surviving a SWB by triggering you mammalian reflexes to close off your air passage should you black out.
An excellent source for freediving information is a book written by Terry Maas, "Freedive!" http://www.amazon.com/Trident-FREEDI...sporting-goods
Make sure you dive with a buddy