I was answering a post that went awol on blendshapes with falloff…

The Blendshapes SOP doesn't support falloff weights. It only knows how to blend between target shapes. I suspect that the question was how to pull a point on one of the inputs and have that affect a radius on the final shape.

Here's a file that shows a couple ways to do this. One on each individual input pre-deforming the input then one that applies attributes before the blendshapes then applies the deformation weights after.

Blendshapes only knows how to blend a few default attributes. I picked on Cd mapping effect strength and effect radius in to Cr and Cg respectively.

Cr = metaball weight
Cg = metaball size

You can paint in to those two attributes and put the display on the final SOP to see the effect.