This video is only to describe the problem with retopo tools in Blender. This is a 2.5 WIP version.
The problem is the visibility of the retopologized mesh. Vieving the video should be pretty obvious but in case not: There's no oclusion by default when you are retopologizing a mesh. You can activate the usual oclusion mode for this cases, but this mode also hides the retopolozing mesh, so working with this mode on can be somewhat challenging to say the least. Many people seems not to understand this problem, so i'm uploading this video. Note that this is more an anoyance that anything, but in some cases can be a real pain to retopologize a mesh. Workarounds exists but usually these are time consuming or more anoying than the problem itself. YMMV.