Well you don't really need 35 passes for free space erasures (it'll take a lot of time) one pass should be sufficient (http://bbs.heidi.ie/viewtopic.php?f=35&t=5394). You can follow the instructions outlined here if you really need to use 35 passes. If you cancel a free space erase there will be portions which have been left unerased; parts of the drive which were erased would be erased completely (all the requested passes) except the portion which was being erased when you clicked cancel.
As there had been 10 passes then all the free space was erased then (it did take a long time). Yes, one pass would have been enough but it was my first time and I wanted to see how long it would take. If one stops after 2, 3 or 4 passes or as desired this will then presumably do the job completely. 35 passes does seem excessive but more than one might seem desirable if erasing sensitive data.
Your disk is erased a portion at a time so that applications won't have to deal with a full disk for too long. When you stop the erasure, there will be portions which are left untouched and portions which were erased. The portions which did get erased are completely erased. There will be portions which no erasure has occurred at all. I don't know but maybe in future this behaviour should be changed to erase the entire disk at once.