This method runs through every player that is all in and adds a value (allin-value) between 0 exl. and 1 incl. to the result. E.g.:

If a player had 100 to call and went all in with his remaining stack of 10 the value for this player would be 0.1.

If a player had 100 to call and went all in with his remaining stack of 50 the value for this player would be 0.5.

If a player had 100 to call and went all in with his remaining stack of 100 the value for this player would be 1.

The idea behind this method is to be able to better guess the number of opponents your are up against. If you just consider GetNumberOfPlayers you will not get the players who are all in. If you add NrPlayersWhoAreAllIn you have the correct numbre of opponents, BUT since it is easier for someone to call if he has only little stack left, this method helps to better guess the strength of the players who are all in.