Complex sentence: Before the rule was installed, drivers would "race back to the caution"; however, there was a gentleman's agreement not to race, but to slow down and not pass, to allow slower cars to get their laps back.
Split into simpler sentences: Before the rule was installed, drivers would "race back to the caution". However, there was a gentleman's agreement not to race, but to slow down and not pass, to allow slower cars to get their laps back.