Complex sentence: Ward was aware of Rathmann's tendencies as a driver, and allowed him to pass him for the lead, knowing that Rathmann would frequently charge hard to take the lead, but once he was in the lead, would often back the pace down.
Split into simpler sentences: Ward was aware of Rathmann's tendencies as a driver, and allowed Rathmann to pass him for the lead. Rathmann was known for charging hard to take the lead, but once he was in the lead, would often back the pace down.