The general idea of all ERGObrass supports is based on a simple fact that all brass instruments are heavy to hold. We all know that relaxation is very important in brass playing. However, holding the horn in playing position inevitably produces a static tension in hands, shoulders, upper body and throat. Most players can play with this tension, but it never make us feel and breath as efficient as if there would be less strain in arms!
It is so natural, that it is much more comfortable and efficient to play when not having the whole weight of the horn in your arms while playing!