It's Doriki.
Doriki is a unit of measurement used by CP9 to directly determine someones level of physical power.
They were heightening this element. It is directly related to the physical strength of a person, and if a person who has high Doriki uses a Haki attack it will be amplified.
Koby amplified his Haki-based attack with the element of Doriki.
Does this make sense?
They didn't use Haki or Devil Fruits because it distorts the estimation of Doriki a person has.
A common armed soldier on average has around 10 Doriki, and according to Spandam, a Doriki of 500 or higher is classed as superhuman.
Koby's Doriki resembles Garp's. So, I suspect Koby's Doriki count is like idk... Over 9000?
Here are examples of CP9s Doriki count when they were at Enies Lobby.
It's actually fairly reliable to suggest Koby's Doriki count is around 9,000 meanwhile Garp's is more like 12,000 or more.
Mathematically, it can go like this...
Koby equals at least +500 more than Rob Lucci did at Enies Lobby. So, we can suggest he is worth 4500 points. Then, you have to consider the element in the equation that considers Garp. His feat that resembles Garp's strength grants him bonus points in relation to how much it would take to knock Koby out at that time which is 4501. So, it becomes 4500 +4501.