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?