Complex sentence: In some rebreathers, one-way valves direct the gas through a "loop", while in others the inhaled and exhaled gas goes back and forth along a single tube: this is called the pendulum system.
Split into simpler sentences: In some rebreathers, one-way valves direct the gas through a "loop". In other rebreathers, the inhaled and exhaled gas goes back and forth along a single tube: this is called the pendulum system.