Never really done it before as I let the Coaches call the shots here.
For example If I have a CB set to 100% Man and 40% Zone and change it to 100-100, what would happen?

So Is there anyone here who has messed with player weights? I'm hoping to get some opinions on what you do, or what kinda works..You can send a PM if you like.

It just changes the way your players are rated, it doesn't change anything about their performance. In your example, you have a CB with the following Man and Zone (assuming that was the only influence in his rating):

Man: 79/87
Zone: 30/87

If your weights are 100% Man and 0% Zone, the rating you see would be 79/87
If your weights are 0% Man and 100% Zone, the rating you see would be 30/87
If your weights are 100% Man and 40% Zone, the rating would calculate to 56/87
If your weights are 100% Man and 100% Zone, the rating would calculate to 54/87

etc. The point is that you can tell the calculation what you feel is important for each position so you can trust the ratings you see more.

Lost in the calculation,JD. I'm not a Math guru, lol..so where is the 87 coming from? It seems to be based off of that number.

The 87 was the future ratings in my example ... I made them both the same in my illustration to show that if two values are the same then it won't change the overall rating if you adjust the balance of those attributes. But the current ratings change based on what you set for the attribute weights.

Note again that no matter what, your CB will play better at Man than Zone in this case, so if you play a lot of zone and not a lot of Man you would want to reduce the weight of man and increase the weight of zone so it's easier to find CBs who have a good zone coverage attribute.

To add to what jd said, you set your weights to emphasize or emphasize the player's skills and attributes that you value the most or devalue the most. This will allow you to more quickly spot and evaluate players that fit your scheme of play - or even find just one player that will most help your team, if that's your goal. It's also very helpful for inserting personnel in certain formations or under certain conditions when used in conjunction with scouting your opponent.

Thank you-
...thought you were referring to something different in your example, got it now.

I experiment with player weights all of the time. I have two sets of weights that I use frequently.

1) Core Skills - Empasizes the skills that you want a player using 80% of the time (so for an RB this would be SP, AC, Carry, and Avoid Fumble) and either doesn't weigh or 1 points all other skills.

2) All Around - Weighs a lot more skills, but still differs widely from the default weights.