Effective Stack Definition:
Effective Stack is found in the filters section which you can access by clicking on the filter button and choosing edit and youll see the filter in the main filters TAB.
Effective stack size measures how much of your stack is at risk when your making your first action preflop.
An Effective Stack (or effective stack size) is the smaller of the stack sizes between two people. The term reflects the fact that, when a larger stack goes against a smaller stack, only the amount of money in the smaller stack is really at stake. For example, if a $100 stack pushes all-in and is called by a $20 stack, each player will be risking only $20; the excess $80 will be returned uncontested to the bettor.Folded to Big Blind Exception:
If your in the big blind and its folded to you then 0 of your stack is at risk so it will not show any of these hands in the filter. If you want to view these hands you could narrow them down by choosing a filter to show just the big blind and set the effective stack to less than 1 and it will return all these hands in the big blind where it was folded to you.Limpers: Other scenario's of note include situations where someone limps early position preflop there stack is counted as part of the effective stack calculation.
Multiway pots: Finally in multiway pots if there are 2 or more players the effective stack is calculated based on how much of your stack is at risk so if player 1 is 20bb and player 2 is 100bb then the effective stack is 100b not 20bb because 100b of your stack is possibly at risk.