This article explains how "Open in Internet Explorer" Safari extension works.
1. In case your Windows Virtual Machine is running:
On clicking on button in Safari browser Internet Explorer will open in running Windows Virtual machine and the same web page will show up there.
2. In case several Windows Virtual machines are running at the same time:
Internet Explorer will open in all Windows Virtual machines
3. In case you have only one Windows Virtual machine and it is not running (Stopped, Suspended):
The Virtual machine will start, then Internet Explorer will also show up opening the same web page as in Safari.
4. In case you have several Windows Virtual machines and they all are not running (Stopped, Suspended):
Parallels Desktop will warn you that no Virtual machines are running and ask to launch at least one of them then try again.