I don\\\'t think a phoenix eye to the side of the head would be likely to kill anyone decent at grappling in a fight on the ground unless they were very unlucky or you had a very dominant position. Consider the difficulties in striking hard when you aren\\\'t standing up and are being controlled by someone\\\'s legs. Remember that elbows to the head are often used in sports matches and there aren\\\'t many deaths from these.
It is all in the position you are right. However, because of the bone line up it\\\'s not that difficult to get destructive power into the punch if you can move through your torso. You don\\\'t necessarily need your legs free, they can be tied up. Also, many times if you can elbow, you can punch (especially for people like me with very little reach) and I would punch instead of elbow everytime.
edit: and I didn\\\'t say likely kill them, I said potentially kill them... usually it is made \\\"illegal\\\" because of the potential, not because of the likelihood.
Can you tell me a bit more the particular bone line up which would make a PE fist from someone\\\'s guard a strike you would prefer over an elbow or normal punch from the same position? Do you feel that most MMA fighters don\\\'t move through the torso enough when punching from in someone\\\'s guard? Do you feel you could deliver a more damaging strike with PE from in the guard than with elbow from rear mount or side control? These blows are seen quite often in comps and there aren\\\'t many deaths.
Come to think of it, PE strikes are probably quite legal in some MMA comps, provided you can form it with the small glove on. Maybe someone should try them?
Properly formed and lined up Phoenix eye puts all of your generated force into a compact area. Think about it as somewhat similar to a ball peen hammer. Your index second knuckle bone lines up all the way through your wrist and lower arm making for one big strong hammer.
It doesn\\\'t take much through the torso to put alot of power behind it, even in stand up where you are fully mobile you are only moving a couple of inches. If you want to have an easier time of throwing the punch move against it to begin with, the automatic reaction from your opponent is to want to pull you back... when you feel the pressure to go back - make the hit - he is not only making it more difficult for him to counter it, he is actually HELPING you generate the power by pulling you in that direction.
Here is the danger in phoenix eye that gets it made illegal - you connect with a properly formed phoenix eye on the side of someone\\\'s head and your knuckle acts exactly like that ballpeen hammer. It can put a nice little hole in their skull and show very little to the outward observer. With a well formed and power produced hit the person throwing the punch feels a solid hit but really can\\\'t know the extent of the damage. The person recieving the hit may have a hard skull and it was \\\"just another hit\\\", they could become an instant \\\"walking wounded\\\" and feel fine until they pass out because of the concussion or they could just go unconcious or worse.
Phoenix eye is also good for striking vital areas and breaking ribs. The concentrated force does nasty things.
Elbows can be fair weapons if they are conditioned to be and practiced properly. The major problem with them is that most people (myself included) don\\\'t practice them enough so they are a \\\"back-up\\\" weapon when you are down to throwing any piece of junk you can think of at the person. The surface area for connection with an elbow if you want to compare it to a punch is pretty bad actually. Take a look at your arm while you are punching. Your elbow hitting area is not moving like your fist. When you punch you don\\\'t fully extend your arm because your elbow dissipates some of the shock and allows you to hit harder. Just that alone should tell you that you have less useable power in an elbow than a fist. To use your actual elbow to hit means you have to sacrifice power, range and flexibility and accuracy to make it connect in a reasonable way. All in all, I would work my punches any day of the week.
Course if you are just looking for something \\\"cool\\\" that looks nasty and speads your power over an area - go for the elbow. It will impress the non-strikers with how \\\"powerful and brutal\\\" it is.
To answer some of your questions in short concise answers. The bone line up I listed earlier... the knuckle bone is solidly in line all the way up your arm. Makes for no loss of power, it\\\'s all one big transfer device with a very small delievery point.
I explained the difference between elbows and phoenix eye fist as for toher punches it\\\'s because of the nature of the fist. Phoneix eye is destructive and concentrated, if I want to get the biggest bang for my buck in one shot, that is the fist I would likely use. If I get stopped with the phoenix eye, I have them distracted and come back with something like a ginger fist from a different angle. Ginger fist will do less destruction into a person but it can give you a nasty cut and is still a good solid hit. Why phoenix eye... simple, you don\\\'t use a pair of pliers to hammer in a nail and then wonder why it is difficult, you use a hammer and save your energy for something else.
Do I feel that most MMA fighters don\\\'t move through their torso enough on the ground? Is that a real question? Have you watched them? Most of them don\\\'t move through their torso enough standing up! Watch them - they make me think of that old kids toy \\\"rock\\\'em sock\\\'em robots\\\".
When you are asking about rear mount and side control are you talking when I have the position or when someone has the position on me?