Here's his breakdown from last year. I remember the dude well because in fantasy my friend and roommate had LaDainian and kept getting PISSED because Chatman was stealing all his carries.
http://sports.yahoo.com/nfl/players/6189/situational
Basically, the bulk of Chatman's carries came in the 3rd and 4th quarter, while ahead, and on first down. 61 of his 65 carries came while ahead of the opponent, 60 of his 65 carries came in the second half of games, and 30 of his 65 carries came on 1st down, with 24 coming on 2nd down.
He gained 6.0 yards per carry to the left, 5.6 over center, and 7.0 to the right, with 60 of 65 carries falling into one of those three categories. Only 5 runs were wide left or wide right.
The normal accusation for a change of pace back like him who gets 6.0 yards per carry is that they did it a lot on 3rd downs, or they did it a lot on pasisng downs or when behind.
In his case, only 10 of his carries were on 3rd down, with only 2 of those being on third and 8+ yards, and those third down carries actually dragged DOWN his yards per carry, since he only gained 3.3 ypc on his 3rd down runs.
As for obvious passing downs, with the overwhelming majority of his carries coming in the 2nd half while the Chargers were up, I believe it should have been fairly obvious that the Chargers were running out the clock and therefore running the ball more.
On the other hand, he was not used on goal line situations much, with only 5 goal-to-go runs with 1 touchdown. But, he did have a 4th and 1 run for 22 yards.
So the dude ran through the center of the defensive line, on 1st and 2nd down, with the defense knowing that the Chargers were trying to run out the clock, and he gained an average of 6.0 yards per carry doing it.
Either the San Diego offensive line was the best the NFL had to offer last year (possible) or this dude's a pretty talented running back.