After a quick read of The Cultimate Opinion blog on defence, I thought I'd put in my two cents. As a disclaimer, I'm really quite inexperienced, so most of this is theoretical and largely based on a probabalistic approach without a lot of data behind it.
At worlds, the stats were entirely for offence related activities, i.e. throwing a goal and catching a goal. There was absolutely no mention of turnovers or d's. It makes it hard to determine the effect of various players on D. Some ultimate statistics would be a very handy thing indeed.
D can appear very unrewarding. My time on Karma (on the D team) was filled with calls for getting blocks and layout D's, and as I really wasn't getting any. I wondered if I was doing a good defensive job for the team. Was I really contributing? I looked at why I wasn't getting the blocks that were called for and I realised that it was because my player wasn't getting the disc thrown to them much.
Being one of the chumpiest players on the team, I was marking up on the lesser abled opposition players. By playing good defence on them, staying as close as I could and not letting anyone make any decent cuts, I was preventing them from getting the disc. At one point I counted 5 points in row where the player I was marking did not touch the disc. Instead of berating myself for not getting turnovers for the team, I started seeing how much I could stop my player getting the disc. I figured if everyone did this, the stall count would go to 10, or there would be a hail mary huck and we'd get a turnover.
I would like to pose a question. Which is the better D; covering your defender so that nobody can pass to them, or letting them get a look and then getting the D? I could also put it another way. The stall count is on 9, the handler has two options, player A who is very well covered by player X and player B who isn't quite as well covered by player Z. The handler picks player B and player Z gets a D. Who played better D, player X or player Y?
I think the type of D, whether it's going for blocks or shutting down your player is largely dependent on who you are marking, and your team strategy. Defense is as much a team activity as offense is. In the above example, what got the turnover was really the combination of the two players. One player helped to force the offense to throw to a risky option. While running a defense in a man-on-man setup, the team should be identifying the matchup which is most likely to generate a turnover. That player then hangs off slightly while the rest of the team cuts off any other options. Hopefully this results in a turnover.
When things are less organised, I find that it can be easier to get a D on an elite player than someone less skilled on the opposing team. If there is an elite goto guy on the other team, I find that the lesser players will often throw riskier throws to the elite player, making it easier to get a D, whereas when you are marking a lesser player, they don't get the disc unless they are wide open. Furthermore, I find that when comparing throwing and recieving, most people are stronger receivers than throwers. If an elite player is throwing to a chump reciever, the probability of a completed pass is higher than a chump thrower to an elite receiver.
To summarise, in a man defense I think the best strategy to generate a turnover is to put the best defender on their best receiver and wait for one of the lesser abled players on offense to get the disc. At this point, shut down all options for them, but leave the elite player slightly open. Rely on the chumpy player not to put in the perfect throw and get the turn. Simple.