I had a quick look just now and the only way it seems to operate is with continuous free run mode, or stopped. i couldn't find a way to trigger it externally. It's not my main SA so I don't use it a great deal as an SA, so it's possible I've missed something, but the settings in SA mode are pretty rudimentary.
Obscurely you can enable the scope's trigger menu while in SA mode, but it doesn't affect the SA sweep, I did try.
There is a non-changeable menu item "Spectrum Free Run", I don't know why it's a menu item, you can't do anything with it as far as I can tell, but to be fair the UI is a bit of a dog's dinner so I might not have the right telepathy rating for changing it.