Watch mainsprings are small enough that aggressive winding can break the spring or the barrel hook where the spring is attached. The brace in the barrel is an attempt to prevent that. Atmos is basically a self winding watch. The motor winds the mainspring whenever the temperature drops whether it needs it or not. The current clocks are designed so that the motor spring will reach an equilibrium with the mainspring, so the mainspring will never be completely wound tight. I imagine they put the brace in there because the early Reuter Atmos could be wound by hand and the mainspring damaged.