Here you will find a basic, but not exhaustive description of Einsetzen/Ansetzen lip positions and double-pedal-note exercises. Another similar explanation can be found here.
The terms Einsetzen and Ansetzen are words that originate in ancient french horn technique and describe two embouchure types that were in common use in the past. As french horn players have such a large range of notes at their disposal it is normal for them to specialise as either a high-horn or low-horn player. Orchestral horn parts follow this tradition by having parts 1+3 as high-horn parts and 2+4 written for low-horn players. In the past it was common practice for a low-horn player to use the einsetzen embouchure, which requires the unrolling of the bottom lip to allow the mouthpiece to be placed on the inside of the lip. The ansetzen position is the more usual way of placing the mouthpiece on the outside of the lips and was used by high-horn players.
In his journey of discovery Jerome Callet found that by adopting the einsetzen lip position he could play clear, in-tune, notes two octaves below low C-F#. This in itself may not seem very important, and it is a very different approach to pedal notes that is practised by more traditional approaches. By themselves the einsetzen-positioned double pedal notes only have a few uses:
- Relaxing the corners of the mouth to allow the obicularis oris to do its job
- Encouraging blood-flow to the lips (good for active recovery on the gig)
- Developing a feel of the top lip pushing down against the tongue and unfurled bottom lip.
However, when you learn to slur or slide from the double-pedal register to the normal playing range by moving from einsetzen to ansetzen then new things are realised. Specifically the interlocking of the lips in an isometric contraction that is very resistant, allowing for a strong upper register. As it is extremely difficult to explain how it feels to be playing correctly this is a very effective method for enabling a player to discover for their self what a strong embouchure set-up feels like with no stretching, straining, or excessive mouthpiece pressure.
These are exercises that were originally intended for use with trumpets or cornets. They also work on the B-flat side of a french horn. As for trombones and larger brass, substituting normal pedals (as these exist in your harmonic series) will be useful, but do not have the same effect. All other aspects of TCE work well for larger brass.
Real understanding of this concept can only be attained by giving it a try. Below you can download three exercises inspired by the original Trumpet Yoga warm-up. They are in no way a complete explanation of how to practise Einsetzen/Ansetzen, but a good starting point for the curious.
Maintaining the tongue through the teeth in the movement from einsetzen to ansetzen is key to feeling the correct sensation for the tongue resting on the bottom lip as it rolls back inwards to the ansetzen position.