When a sync is cancelled, the temporary file created on the destination with the data copied so far is removed.

When starting the sync again, the bases are compared again. After the comparison is complete, the sync will effectively resume from the last successfully copied file; the files that were successfully copied before cancelling the sync will not be copied again, as they are now equal (unless the overwrite policy is Always).

If you want to instantly resume a sync operation, you can pause it instead of cancelling it.