Let's see if my memory is right and I'm sure I'll get some of this wrong (been a while since I played with this stuff) But when you simply click del it doesn't actually delete the files but marks the sectors as writable so the info to be deleted get written over, So deleting really doesn't delete the stuff but marks the related data to be written over. This helps the disk live longer also because each delete if it actually deleted .the file would be writing data over it so it would work much more, Kill disk rewrites the data on the drive instead of just marking it as open space so it replaces all the data with zeroes. If I'm remembering right I think that's how it goes
As for digging up data, when its marked as writable new files get written but some files may not get written over and will remain on the drive until it is written over, most people don't fill a HD so some files deleted may still be there or part there and the companies who retrieve it try and repair the files if there still there, Otherwise it's gone and new data is there.
Dauntae