Well, technically it's not actually "pinching" per-se. I initially starting the implementation by calculating a vector from the center of the brush to the nearest point on the curve. Then for all nodes inside the brush (the area of effect), pull the node towards that vector. In practice, this was not so useful to work with, and the result was not what one would expect, though that's subjective I guess.
Instead, the pinch mode currently works in a simpler fashion, but gives better results in my experience. The nodes are simply pulled towards the center of the brush. The "falloff" of the brush is amplified in this mode, so nodes closer to the center are affected exponentially more than nodes further away.
The pinch mode can be used to create sharp corners, or to tighten the curvature quickly. It's engaged by pressing ALT while using the Manipulator Tool.
With the recent closed Beta release, you can try it yourself and provide feedback. Kindly join the beta program and download Curver. It's as simple as sending an e-mail.