Most times, a lifeline is used to show that one friend is in distress, when he's actually chillin at home with two beers, one in hand, and one waiting for the other friend. The distress call is what gives the friend his opporunity to leave, and also gives the impression to whoever is litening to the phone call that it's serious.
The lifeline remains the most efficient, yet most underused method of ditching bad dates. Sometimes hard to coordinate, lifelines prove their worth in effectiveness when your friend's girlfriend doesn't get pissed (and consequently, stop putting out) at your friend when you ditched her best friend on a date.
This person will always have that six sense to lie in favor of you when asked about your current location (sometimes without you giving them any real hint about it).
This person also happens to be in the right place at the right time when you're in trouble.
"Thanks, man! You're a lifeline!"
In situations where you're running late getting into work:
Boss: "Do you know if your name came into work today?"
Lifeline: "Uh... Yeah, he came in an hour before I went on lunch break. He's somewhere around here."