I tried to answer this, but I really struggle coming up with a coherent post.
Se is weird to me. Its like I TRY to use it, but it gets trumped by my other functions. This also makes it difficult to describe. I'll try my best (not even sure if this is Se):
Extraverted sensing is the way I feed my brain. Its like the mouth to my intuition. I don't use it primarily, instead I just "dabble" to understand something conceptually. Its an abbreviated way of interpreting the world.
For example, with hobbies - it seems like a good idea to read a book... I'll try but I won't finish it. I'll get the general idea of what it is about and I'll get uninterested in it (sometimes just going to Wikipedia to find out the rest of the plot/symbolism). I can play a ton of instruments, but not well. I get introduced to them, how they work, but then stop when I feel like I get the gist of it. I learn through experience - hands on style... but just to get acquainted with something. I have no interest in learning something fully. Just generally. With cars for example, I know a ton about cars in theory, but pop the hood of a car and I can barely tell you what is what. Same with fish and aquariums, etc. I just acquire very board hobby type knowledge and then move on.
Same thing goes with the way I perceive things. I scan things to feed my intuition. My intuition interprets what I see and fills in the blanks. While a Se would notice details/trees in the forest, I browse, nit-pick what I view as important, omit the rest, and use my "sample" to assume the forest, what it has been, and what it will be.
I believe Se is used to expand the function of Te/Ni axis so that it can be applied to the immediate physical world.
As ENTJs grow older, they'll find that they become more involved with their surroundings. They slowly develop the ability further to apply the knowledge that they've built up on a broader scope of things.
Also, Se brings the data to Te/Ni. The more Se develops, the quicker (and more broadly) it scans the environment to find useful information. This perhaps one of the reasons why ENTJs work and think rather quickly. It's like skimming through a text looking only for the important stuff... only you're not necessarily concentrating.
I don't know when exactly my Se shines in action. I'll think about when it springs into noticeable action and report back later.
If we look at things in terms of perceiving functions and judging functions, Se becomes much clearer.
Ni/Se can be split up between
"Thought" and "Action."
"Caution" and "Risk." "
"Future" and "Immediate."
So on. That is, of course, a general view. But when an ENTJ is in an environment/atmosphere whereby their Ni becomes less useful in being able to figure out the immediate world, Se will give them the ability to interface with the physical present in its full capacity. Take a running race for instance. Five, four, three, two, one - Ni is no longer useful - GO!
Essentially Se is being able to be fully absorbed in the real world without those niggling thoughts that Ni gives you, without those questions that it brings up in your head or those abstract mind-trips that last for ages. Ni may on the whole be more dominant than Se, but I think that as an extraverted type, Se comes naturally to the ENTJ - for them to be "in the present" is a natural feeling.
Lets look at a stereotypical way of looking at it. Look at some of the stereotypical careers of ENTJs.
Army Officers: While Ni might be useful in operational planning, Se comes to the fore when you're in the field: when there's no time to think X Y or Z and you just have to do things and do them quick and competently. You have to be there. You have troops to lead, orders to give, support to call in and objectives to reach and occupy. No time for dilly-dallying: the time is for action.
Trial Lawyers: You're up in front of the Magistrate. It's your turn to speak. Twelve members of the Jury looking at you, a Judge in his wigs, the opposition barrister looking calmly to see what you are going to submit. You don't spend time stuttering and er-ing and uh-ing. You just get in there and do your job. Embrace the moment of having your say and saying it well.
Businessmen: Negotiations are ahead for a merger. ENTP Inc has put up its best negotiator against you. Are you going to sit there dryly listing of statistics and performance data of your Company or are you going to get into the moment, the very physical reality of what is around you and start pushing your side of the bargain?