it's a round based bet game (6 players)
you bet on the first byte of the 6th players block hash (unpredictable, 50% chance)

to join:
send one byte data (0x01 or 0x81) with a bet amount of 1 ether to 0x0312855bb6e548578a36fa8de63b8f0d3d7d7dc2
 - or -
send one byte data (0x01 or 0x81) with a bet amount of 0.1 ether (100 finney) to 0x7753a9d834844cfde5c211ec3912b49f0d8b8e11
 - or -
send one byte data (0x01 or 0x81) with a bet amount of 0.01 ether (10 finney) to 0x615d2c5155ea9841f2a926c3a4953d140d407c99

(click address to check the verified contract)

if your data byte is less than 0x80 you bet the 6th players block hash first byte is less than 0x80
if your data byte is greater than or equal 0x80 you bet the 6th players block hash first byte is greater than or equal 0x80

if you lose your bet your bet amount goes to the pool for winners

if you win your bet:
   * you will get back 100% of your payment
   * you will win a proportional part of the winner pool (win amount = winner pool / winners - 1%)

payout is triggered when a player starts the next round

additional rules:
each address can only play once per round
every additional payment during the same round will be paid back immediatly
every payment below the bet value is considered as a donation for the winner pool
every amount that is exceeding the bet value will be paid back
if nobody wins in a round, the paid amounts will raise the winner pool for the next round


how to send a data byte together with Ether:

If you are using Geth or Ethereum Wallet (wich starts Geth in the background):
after typing in a command line, enter:
to send "0x01" and 0.1 Ether to address 0x7753A9D834844cFDe5C211ec3912b49f0d8B8e11.

Change "0x01" to "0x81" if you want to change your bet or change value and address if you want to risk more or less (see original post).

In the GUI of Ethereum Wallet (Mist) just click SEND and after entering the address and the amount click the ADD DATA button and enter 01 or 81 in the input field below the button.