What is the logic based on which resource leveling is done?

[Sailesh Babu]

When I do resource leveling, one or more of the activities scheduled for an over- allocated resource is moved. On what basis does Ms Project decide which activity to move?
What will happen if two activities of same duration, same successors are to be levelled. How does Ms Project prioritize the activities to move them?

Thanks in Advance,

Sailesh