In this mode Ableton Link acts as a MIDI slave to an external MIDI clock by manipulating the Ableton Link tempo to keep it in sync.
Select your MIDI source.
At this point, MIDI Link Sync is listening for an incoming clock, but it won't touch the Link timeline.
If you want to test the connection, go to the source hardware or app and start it. Then come back into MIDI Link Sync and you should see the MIDI section showing a tempo and a scrolling timeline. If you don't, you should check the routing of your MIDI clock. Don't forget to stop your MIDI clock after doing the test!
When you're ready, press 'Enable sync'. Once MIDI Link Sync receives a MIDI clock start message, it will start adjusting the tempo of the Ableton Link timeline to bring it into line with both the beat and tempo of the MIDI clock.