can anybody say what is the technology used behind data recovery in the system

I file is never completely deleted.Some traces of it are present on the HDD.You can recover the data by using a software called "Recover My Files".

According to me, a HDD is made of many sectors. When we save data in a HDD, the data is stored in this sectors. But in a random way & not in a sequential way. Moreover, it is stored so that it can be recovered anytime evenif only one sector the data stoed is left. This is none other than our advancement in technology.

As far as I know the data is never completely removed from the hard drive unless it is physically destroyed or the hard drive is overwritten many many times, instead the file system is changed. So what is gone is the road map so speak. So there is software that can bring up the data and then piece it back together something like someone painstakingly piecing shredded docuements back together like a jigsaw puzzle. Both are very difficult but not impossible.

well all i can say is when a file is deleted ,the link to that file is deleted from TOC (table of contents). all that is lost is tiny bit of information that tells where the file is located. But after the harddisk writes new information over the location of that file previously deleted, its bit difficult to recover but not impossible.