Ummm..

For classical guitars, where the neck is much wider than electric guitars and western-style acoustics, it is important to have your thumb right underneath the fretboard. I can't see the specific lesson you are talking about but I assume he is instructing to have your thumb on the back of the fretboard, in the middle of the neck. This is so that you can reach all the strings correctly and maintain a proper position of your fingers when playing a "classical" guitar. This rule doesn't apply to playing an electric guitar in my opinion.