External trigger back panel connection is handy in rack
setups, put the master timebase to it for best jitter and
timing measurement consistency. Or loop your 50-ohm sig
gen source to it as well as the DUT, rather than using a
probe. It's just a resource, there in case you need it.
If two events like pulse of your interest and the pulse causing it are time correlated, the external trigger could be used.
Y input can be your signal and the external trigger could be the pulse initiating the process.