In reply to mypyrex:
As the others have said: the live signal is put straight on to the hard disk then read off and shown to you. Depending on the unit this can be anywhere from half an hour to an hour. The writing and reading from disk are two separate tasks so can be controlled separately. When you pause live TV you are stopping the reading bit, when you resume then the writing and reading are now separated by however long you paused for. The unit doesn't "catch up" unless you skip forward in some manner.
This also means that if you have been watching a programme from the start and then decide to record it then the saved recording will start from the beginning so long as you aren't beyond the half hour or hour buffer length.