Oh Ok just cuz its currently christmas and Im wearing Raindear horns on my head and Im happy.
;D
Basically it puts the game into slow motion, like bullet time in Max Payne, except it has no time limit on how long you use it for.....its so cool....you can see bullets from Machine Guns coming at you and dodge them.
You used to have to access the Slo Mo by opening the console and typing in some cryptic command line, but it became so popular that the developers decided to include it in the latest patch.
Another interesting side effect of it is that when you shoot someone with a powerful gun, like the sawn off shot gun they will fly to the other side of the room in slow motion, but not at normal speed....you can do heaps of cool thing like shoot garbage cans so high in the air that they dissapear into the sky..
If you want to see some of these cool effects without buying the game then goto
http://www.3dactionplanet.com/hitman47/pota/ and take a look.