Thanks for the explaination. I now understand what it does. The confusion was caused by the term "Layer". I was thinking of a layer in terms of a separate map layer as in Google Maps where you can enable and disable the map view layer on top of the sattelite view, or where you can add and remove a "My Tracks" layer on top of a map.
Here is an alternate way to describe the function of the lock button: "The lock button prevents the zoom function from loading new maps as you zoom in/out. Instead of loading new maps, the zoom function will simply enlarge or shrink the existing map. This may be useful if you have offline maps, do not have a particular zoom level and still wish to zoom in."
