Penalty kicks in FIFA 10 haven't changed much from FIFA 09, with really only one little tweak. Because fouls are so frequent in online play in all versions of FIFA, you ought to know how to both save and score penalties. There are three ways to score penalty kicks:
To execute a regular penalty, press the same button that you use to shoot normally on your particular gaming console. The best amount of power to use is just enough so that you can see a bit of red, but not a lot. Experiment in the arena to perfect this. Use the analog stick to aim, you don't have much control over anything but left or right with the analog stick, the power of your shot determines the height and of course the power.
To execute a placed penalty, hold down RB on Xbox as you press the shoot button, and R1 for Playstation. The placed penalty supposedly has more accuracy than the regular driven one, and thus less a chance of missing the target. Other than this, follow the same principles as given for the driven penalty tutorial.
There really isn't much use for the chipped penalty other than to be a bit cheeky. Only try to do this if you are quite sure that your opponent will make the keeper dive one way, as you will look stupid if you try it and it chips up right into the hands of the keeper. To execute a chipped penalty, hold down LB as you press the shoot button on Xbox, and L1 for Playstation.
I know I said there were three ways score penalty kicks, and you might think that this makes it four. However, this really isn't a method of scoring penalties, as it occurs on a completely random basis, and can only be done with Cristiano Ronaldo. The stutter step penalty is when a player is running up to take the penalty, but suddenly stops to startle the keeper, and then goes ahead with the penalty. It is generally useful in real life, but in FIFA it doesn't really fool the keeper.
Saving penalty kicks in FIFA 10 hasn't changed from FIFA 09, the general concept is to either dive or stay in the middle. Now, the best method is to guess the direction of the shot, and dive that way or stand if you believe he is going down the middle. Remember that when diving, you cannot just dive left or right and expect to save the shot if it is going that way, but rather you must also guess the height of the shot.