The actual EDA sensor is on-demand only. You put your palm over the screen so it touches the metal ring around the watch and it essentially measures moisture on your skin to detect stress. You then get a little graph showing how many "responses" you had - lower is better.
The Fitbit app also includes a "Stress Management" score. This pulls data from all over - heart data, activity levels, sleep quality - to give you a score out of 100. It's all a bit vague but you can see the areas you are low in that need work. And of course, just taking the time out of your day to perform the EDA test will help with mindfulness and relaxation, regardless of what the "score" at the end is. A couple of minutes doesn't sound like much, but it's a surprisingly long time to just sit and relax.
