In addition to the "float" issue, note that most pedals including most Shimano mtn bike "spd" pedals allow you to adjust how much tension is on the release. People new to pedals may want to back off the tension (normall a very, very small allen wrench is used near a spring on the pedal) so you can pop out with very little effort. If you ride aggressively, standing, sprinting and such that may not be a good idea as you don't want to come out of the pedals unexpectedly but it's something to be aware of and understand if your pedals do indeed allow that adjustment.