The roll-back essentially resumes from a bit further back in the partially transferred file, ensuring that the file isn't corrupted by bit-changes at the resume point. The reasons for corruption can be many, but the most common is related to the file-system reporting the wrong size for the file.
Normally you won't need to restart the program to reflect changes to the options, but that really depends on which build you use, and wether you're using any of the newly added options to ävoid immediate changes.
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