Seat Jack: this is simply a way of insuring a persons seat will be taken by you. By calling "seat jack" before a person calls seat check and after that persons but leaves their seat, you have in effect "jacked" their seat.
Different groups of people use different rules so be sure to ask for "The Rules of Your Seat Check" before leaving your seat, when with a new group of people. The number one most important thing about "Seat Check/Jack" is that they CANNOT be disobeyed. The "Check/Jack" when called stays, until the time limit is reached.
Depending on who you are "Checking" your seat with, there will be a time limit. This means after the seat is checked, that seat is only on reserve for normally 5 minutes. So that after 5 minutes without the "seat checker" returning, the seat may be "jacked." Simple right?
Some variations include:
1) not called seat check but "quack quack:" meaning "quack quack, no seat jack"
----called the same way as seat check----
2) many different time limit are used, most widely used being 5 minutes but up to 20 when going to the bathroom, persay.
3) ALWAYS a safe thing to do is
- make sure seat check/jack and quack quack are understood
- then while still sitting, call "seat check quack quack!" or if with a highly educated group call "seat quack!"
- make sure another person hears you call it, look them in the face and say it!
Again, always obey seat check and quack quack, always!
person b: damn
person a: "quack quack!"
person b: yes finally a seat
person c: nope sorry, he called quack quack