This is a java applet, which you need java to use. To download, go to http://java.sun.com/javase/downloads/index.jsp and download the latest 'Java Runtime Environment (JRE)', you do not need Java EE, Java SE, or JDK.
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For preflop computations, the overall percentage is the mean share of the pot that the pocket cards will win if the specified number of opponents are dealt and no one bets or folds. The simulator runs by dealing the specified number of opponents and 5 board cards at random, finding the winner(s), and repeating.
Postflop: Unfortunately, the overall percentage has less meaning for posflop computations, because this program is not taking in factors like betting history and position. The simulator works by dealing 9 random opponents and removing the worst preflop hands using a simple algorithm that estimates the rankings in this table for Hi Lo or another table for Omaha High, the number of folders is determined by: numFolders = 9 - remainingOpponents. The simulator then deals the five board cards, finds the winner(s) among remaining opponents, and repeats.
Note that for preflop, the #opponents select box lets you change the number of players dealt, where for post flop a 10 player game is assumed, and #opponents is used to determine how many players have folded. I might just add another select box so you can distinguish both for all rounds.