The ISRO Chairman has reported that the economic use of fuel in the Orbiter has led to the mission to have an extended life of nearly 7 years. How has been this dramatic increase in the orbiter life achieved?
How has life of the orbiter increased from 1 year to more than 7 years?
The stated one year operational period represented the pre-mission plan. That pre-mission plan had to provide enough delta V to transfer the vehicle from an insertion orbit to a more eccentric Earth orbit, then to a lunar transfer orbit, then to an eccentric lunar orbit, then to a nearly circular low lunar orbit, and finally, to perform attitude maintenance needed to maintain that low lunar orbit for some amount of time (preferably at least one year).
Such plans have to be conservative, particularly when propellant and large orbit changes are involved. Sometimes thrusters provide a bit less specific impulse than expected. Orbit determinations are never perfect. The imperfections in the determined orbits inevitably result in imperfectly performed maneuvers that need to be corrected.
Vehicles always carry more than the nominal amount of fuel needed for these reasons and others. And even then, there have been failures and close calls. Apollo 11 landed on the Moon with wisps (thirty seconds) of propellant to spare. Voyager 1 barely made it out of Earth orbit due to issues with the second stage of the launcher. The third stage Centaur had been loaded with an extra 1200 kg of propellant for margin; it needed every last drop.
In the case of Chandrayaan 2, the same propellant that was used to bring the vehicle from its insertion orbit to lunar orbit will be used for orbit maintenance. Thanks to and combination of good luck and skill, the vehicle did not have to eat into its margin while on the way to the Moon. That leaves all that margin for extended operations.
The lifetime probably didn't increase. Only the estimate increased. Due to uncertainties in any space mission we tend to err on the side of conservatism when advertising mission life; if we are fortunate we later add to the mission. The NASA Mars rovers Spirit and Opportunity were "supposed to" last 90 days or 90 sols (not sure which), we all know how that turned out. Possibly the more optimistic estimate for the Chandrarayaan 2 orbiter is also driven by a desire to counteract the blow of the likely landing failure.