And other works also which require strength, such as the production of fire by rubbing, running a race, stringing a strong bow, are performed without breathing up or down. Therefore let a man meditate on the udgîtha (Om) as vyâna.

In plain words: Other tasks that need strength, like making fire by rubbing sticks, running a race, or bending a strong bow, are also done while holding the breath. So one should meditate on Om as this held breath.