This is the total spin divided by the active spin (or the spin that contributes to the movement of the pitch). Spin efficiency is not necessarily a higher is better type of metric. Certain pitch types will be effective even at lower Spin Efficiency percentages.
This determines how much movement a pitch has in the direction of its spin direction. A pitch with 100% spin efficiency will have the maximum amount of movement that it can have with the pitch’s Spin Rate.
Real life Example:
Certain pitch types like breaking balls are more effective when they have higher spin efficiencies. Other pitches like certain change-ups or a splitter will be more effective with lower spin efficiencies.