Here is a crude example of a magnet on the P4 Casual Womans "shoulder" (upper arm) where its zScale is controled by bending the forearm to give the impression of muscle bulge. Position the magnet on the figure's shoulder. Save it to a a props pallet, using 'Select Subset' to include the zone and base. Open the resulting pp2 in a text editor. Scrole down to channels section for the magnet.

magnetDeformerProp Mag  1:1
        {
        name    Mag  1

Scrol down further till you find the 'propagatingScaleZ zScale' channel. propagatingScaleZ zScale { name GetStringRes(1028,8) initValue 1 hidden 0 forceLimits 0 min 0.001 max 100000 trackingScale 0.004 keys { static 0 k 0 1 } interpStyleLocked 0 }

Add this code after the 'interpStyleLocked' line. valueOpDeltaAdd figure 1 rForeArm:1 yrot deltaAddDelta 0.010000

The first line tells Poser what type of ERC to use. The second line seems to specify the figure (though in my experience anything can go here). The third is the body part that the slave channel will read from. The fourth line is the channel in that body part that it will read from. The fith line is the control ratio, this line sets the ammount that the slave channel will respond to a change in master channel. You will have to tetermin the best value for this last line by experiment. There are some other parts of the channel that you will probably have to modify. Set the value in the 'forceLimits' line to '4' (on). Set the 'min' line to 'min 1' this will stop the channel from going to a negitive value. Set the 'max' line to 'max 2.000' this will determin the maximum value for the channel. Here is the complete channel from the magnet: propagatingScaleZ zScale { name GetStringRes(1028,8) initValue 1 hidden 0 forceLimits min max trackingScale 0.004 keys { static 0 k 0 1 } interpStyleLocked 0

                        }

I have attached the text of of the magnet described above. You can copy the text, paste it into a text editor and save it to a Props folder as "ERC-Mag.pp2" (for example). This is only a crude example knocked togethr in a few minutes to demonstrate the important points.