The visual effect of "bells up"
When the speed and volume of an orchestral piece picks up, the audience also becomes more excited. To express that feeling visually, some pieces or conductors will suddenly call for the horn players to stand up when performing their part. Sometimes, the horn players are even asked to raise their bells when performing. Clarinet and oboe players can be seen arching their backs to raise the normally downward-facing bell, which looks dramatic but also a little bit uncomfortable for them. The horn players also raise the bell of their instruments, which are usually facing backwards. You should see it if you get the chance!