windows 10 – Why is disk cloning online safe?

When last I cloned an HDD, a few years back, the program (I forget which it was…) rebooted my machine to run the clone. I’m now doing this again but all the versions of clone software I’m seeing seem to work without rebooting, which concerns me.

Can anyone recommend a current program which reboots (but NOT off a CDR: my new machine doesn’t have such a beast, so Clonezilla is out), or explain how the cloning program can be safe without doing that?