A large portion of races have +2 to a physical stat, +2 to a mental stat, and -2 to another stat.
For example, Halflings have +2 dex, +2 cha, -2 str.
With this rule, when choosing a race described above, you can choose to move one of your "+2".
If you move the +2 from a physical stat, you have to move it to a physical stat.
If you move the +2 from a mental stat, you have to move it to a mental stat.
For example, Halflings could have +2 dex, +2 int, -2 str.
This can be used to cancel out your penalty.
For example, Halflings could have +2 cha and nothing else.
Either the physical stat bonus or mental stat bonus can be moved, but not both. One has to stay.
The penalty cannot be moved. It can be filled as described above, but not moved.
If an alternate race trait would replace your bonus to a stat, you would not be considered a race with [+2p, +2m, -2] anymore, so this rule could not be used along with that alternate race trait.
The choice is made at character creation, and once it is made it cannot be changed, even by retraining.
If you are using this rule, you may be asked more details in the background of your character.
This rule is meant to encourage creative and new uses of the races.
It should make more colourful and varied characters, as well as break stereotypes about non-human races. Humans are defined by their variance, but members of other races aren't always exactly the same, either.
Also, humans still have the addictive advantage of an extra feat, I don't believe this makes them obsolete.