Say you wanted to find out how many hits Keith Foulke should've given up last year. You goto to the Baseball-Reference team page for the '04 Sox.
http://www.baseball-reference.com/teams/BOS/2004.shtml
Do this for the Red Sox team (they add all the stats up, you'll seem them)
(BFB - HR - BB - SO - HBP) thats how many balls were put in play. Then do ( H / [how many balls were put in play]
Then, Foulkes numbers were 333 BFP, 8 HR, 15 walks, 79 k's, and 6 HBP
(333 - 8 - 15 - 79 - 6) 225 balls were put in play vs. Foulke, and if you divided H / balls in play for the team you would've gotten .325.
225 * .325 = 73 hits Foulke would've been expected to allow, so the Red Sox defense saved 10 hits for Foulke. (Because he actually gave up 63)