Following up on my previous post, the first reason that can cause flautists to cover too much of the embouchure hole is compensating for a lack of air. Instead of focussing on a good consistent airstream that is pliable and appropriate for what is being played, the flautist rolls the flute inwards, overworking the lips, attempting to produce a better sound. The higher up the flute they play the more the flute gets rolled in and the tighter the lips become. This produces a thin tone lacking projection as it reduces the harmonics present in the sound. Intonation becomes very unstable and has a tendency to become very sharp as you ascend through the registers. Additionally an over covered embouchure hole limits the dynamics that are possible; especially at the forte end of the range. So the thing I often tell my students is more air, less face.

Sometimes when talking about articulation it’s easy to get caught up with what the tongue is doing and to forget something quite fundamental about playing.

The tongue produces no sound on the flute. So irrespective of what your tongue is is up to, in order to produce a good flute sound you have to have good air. 

Remember my motto ‘Air is Sound’ or ‘Sound is Air’. So whatever you are focussing on with your tongue don’t forget to blow!