Yes, so what you really mean is that the tactic didn't really WORK until the end of day 3. I knew that if Dhoni played a defensive type tactic the fan boys would be out there defending it as being "not really defensive but really clever". Let's pretend you didn't know that Kumble had retired and you were watching the 1st session yesterday. Wouldn't Joe the Caveman be thinking - geez Kumble/generic Indian captain's being a bit defensive isn't he?? But because Dhoni was the captain, it's suddenly the smartest tactic ever devised
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The fact that Australia imploded before tea had very little to do with the tactics he employed anyway. Hussey and Watson fell to unusual dismissals and Clarke was just dismissed by a great straight delivery that made him play with a 6-3 field - that was all Ishant, nothing to do with Dhoni. So the damage wasn't done by the wide bowling or the 8-1 fields at all. Sure it slowed Australia down, but I think Katich and Hussey would have been reasonably happy to grind out runs for a while longer. They knew Zaheer and Ishant couldn't bowl all day (though Dhoni tried...). It's just that Katich didn't quite make it to lunch.
And just a personal rant but people need to stop using "defensive" as a swear word. Kumble was too defensive people say, and they make it sound like a disease. Captains often need to be defensive, it can save a lot of runs and frustrate batsmen out. But I personally think that the morning of day 3 with a 250 run lead was too early to start being defensive. Dhoni seems a good captain, but he's not a god and everything that happens during a day is not because he's pulling the strings, most of it's luck (Richie Benaud once said captaincy is 90% luck)