Conversation so far:
What is the marching technique?
Given that instrument-wielding members most often face toward the audience (for maximum sound projection), marching technique must not affect the rigidity of members' upper torsos.
How rigid is their upper torso supposed to be?
members of the battery, due to the nature of their equipment, must keep their entire bodies facing forward at all times.
What is another factor of their technique?
Horn players may twist their lower bodies in their direction of movement, but members of the battery, due to the nature of their equipment.
What are members of the battery supposed to do?
must keep their entire bodies facing forward at all times.

Question: Why can horn players twist their bodies?
Self-contained rewrite: Why can Drum and bugle corps horn players twist their bodies?