Ok, so my dad was watching the Yankees vs Red Sox game last night (8/16) and Boston had a man on first & second with one out. The batter hits a slow line drive between second & third and A-Rod manages to catch it for an out (same as any fly ball). But the Boston runner on second, thinking it was gonna be a base hit, had started running to third but turned to go back to second, then A-Rod just flipped the ball to the second baseman with his foot on the bag for a double play. Thing is, there’s no force with the batter already out, so why didn’t the Yankee second baseman have to tag the runner and not just the base for the double play?
Probably because the runner left the base prior to the catch.
This is an appeal play, not a force play. A runner has to re-touch his base when a fly ball is caught, and can be put out if a defensive player with the ball tags the base before the runner gets back.
See http://mlb.mlb.com/mlb/downloads/y2013/official_baseball_rules.pdf pages 70-71:
Essentially the basic rule in whether the base must be touched or the player tagged comes down to this: do they have a choice?
If the runner can turn around and run to another base, then he has to be tagged. If there is only one base he can go to, then only that base needs to be touched. In the above case, the runner had to return to second with no other options, so just tagging the base is sufficient.
Ok, that all makes sense now. Thanks for the replies!