Flying Wedges - Extensor Action

[KevCarter]

Whenever Extensor action is explained, just like on the Alignment Golf DVDs, it is mentioned that the extensor action is not on the plane, but rather under the plane. I see that, and believe I understand it visually.

My question is, what is the significance of the extensor action happening under the plane rather than on the plane as it relates to the golf swing?

Thank you,
Kevin