Good answer below about how to get the mod on the activity... If you're wondering what to do during the actual activity, think of the trade-off mods as getting a good with the bad. Like the Haste trade-off. You regen health as long as you keep moving, but you short circuit if you stand still. And I think there's one about solar/scorch. It adds scorch to your solar weapons, but also allows enemies to scorch you back.