You might find this abstract interesting:
Compression in mammography and the perception of d... [Australas Radiol. 1997] - PubMed result .
In general, the more compression, the thinner the breast and the better the image will be. How the compression is applied may vary with the equipment. When I had mine done, I believe it was with a foot pedal, which allows the tech to use both hands to position the breast. I have had additional views and a needle localization biopsy, and the compression for those was much tighter.
The machine automatically releases the compression after the picture is shot.
An inexperienced tech might be afraid to use enough compression --- she doesn't want to hurt anyone.