The Tumble Definition

[birdie_man]

Is it because the clubhead "tumbles" (from the top) down the face of the plane as if it is coming over the top of a hill.

(down the line view)

??

 

[cmartingolf]

My interpretation: on the downswing, the sweet spot hides behind the hosel until release (+/-), then rotates to the sweet spot plane at impact.