In the Mimicking the Constraining Object Using Nulls section of the docs, it says that you can
…use a null’s size, shape, and location (animated or not) to determine the area of the cloth’s or curve’s vertices that are constrained. All the vertices that are within the null object’s space are used for mimicking the constraining object, which can be deforming. You could then animate the location or size of the null to create different constraint effects.
That sounds easy enough, but the other day a customer reported that he couldn’t get this to work. It seemed that Syflex Mimic Null was using only the vertices within the null at the start of the simulation.
He sent along a nice test scene for us to work with, so we were able to get him an answer pretty quickly: you need to go into the Syflex Mimic Null compound and select the Reset checkbox on the syflexIMimic node.
Do that, and you’ll get this kind of effect, where as the null position and scaling change, different vertices are constrained to the object being mimicked.