The Power to fight the enemy that have higher rank.
The higher the ranking, it becomes more common to find people with higher Heaven Defying Battle Power.
Lets take an example:
i am an cultivator of origin martial rank 1 and my opponent has cultivation level of origin martiral rank 4 but i am still able to defeat him that means i have heaven defying battle power + 3, i can jump 3 rank to fight an opponent. remember if my opponent has similar heaven defying battle power then i wont be able to defeat him.