Your effective stack is the maximum amount of chips you can lose in a hand
The 2nd smallest stack involved in a hand.
Say you are a big stack with 20bb (20 big blind) and there are 3 stacks behind you: 4bb, 5bb, 5bb.
Your effective stack is 5bb.
Playing the big stack
One of the most common situation is that you have a really big stack and there are several short stacks. In many cases the effective stacks are such that shoving any two cards (ATC) becomes correct in every position. This becomes even more true as soon as pay jumps (and ICM) kicks in. Final table bubble and money bubble are a great example. In turbo tournaments it is not uncommon for big stacks to “abuse” those spots, rightfully so.
An easy way to discover those spots is by putting yourself in the big blind and ask yourself, would you have to call according to chip EV if a shorty shoved. If the answer is not a definite NO, you have a ATC shove spot. Even if a shorty knows you have any two, it is hard for him to call, because shoving any two the next hand is often more profitable than calling your shove – yeah I know turbo tournaments are strange!
Rule of thumb
If calling according to chip EV with ATC is almost correct versus most players left to act, it is always a shove.