Complex sentence: Only Twice was Jones witnessed to be in step with the platoon, once after a whole years practice but quickly reverted to form, the next time was when the platoon as a whole took the extra second Jones always does to respond to make them look orderly.
Split into simpler sentences: Only twice was Jones witnessed to be in step with the platoon, once after a whole year's practice; but he quickly reverted to form. The next time was when the whole platoon took the extra second Jones always does to respond to make them look orderly.