The F Tool™ is now available in the Tools section
A well-playing harp has a solid connection to the vocal tract. A big part of what makes it is easy to play, sound nice and fat is that it's airtight.
If it leaks air, the connection between your vocal tract and the instrument is weakened and the harp feels stiff. The tone is also weaker.
The good news is air leaks are fixable! In fact, that's the fundamental step in improving a harmonica. If you skip this step, you will usually end up working against yourself further down the line.
The detailed instuctions on How to use the F Tool are here.