Despite anything anyone ever tells you
a nollie is in no way a switch ollie. It is much harder and you must take precautions when using it. As
Jikuu says, you pop the
nose of your board. Normally, if you go to
ollie an object, your nose comes up first and clears the object, then comes down first on the other side (the same for a
switch ollie). With a nollie, your nose
doesn't go up, so if you try to nollie an object like you ollie it, you will eat it when your nose smacks into it.
For a nollie you must start nollieing very early so that your nose clears the object. Similarly, if you nollie too early you wither won't clear it or you will eat it when your tail smacks into it.