Almost what ockham said. It's a bit ambiguous what you mean by "x position", but I take this to mean xTranslation.

You put the code in the channel that is to be controlled, and the code "points" to the channel that is to do the controlling. Let's take the Poser ball prop as an example. To have yrot react to xtran, you would use this code in the yrot channel:

           rotateY <span style="color:#ff0000;">yrot
</span>                 {
                        name GetStringRes(1028,10)
                        initValue 0
                        hidden 0
                        forceLimits 0
                        min -100000
                        max 100000
                        trackingScale 1
                        keys
                                {
                                static  0
                                k  0  0
                                }
<span style="color:#00ff00;">                  interpStyleLocked 0
                        valueOpDeltaAdd
                                _NO_FIG_
                                ball_1
                                <span style="color:#ff0000;">xtran
</span>                         deltaAddDelta 360.000000
</span>                 } 

This stuff is called ERC, and the code block which does it is called "slaving code". The deltaAddDelta line determines the strength of the slaving. In the above example "deltaAddDelta 360.000000" means that the prop will do one full rotation for every poser unit travelled. To increase the rate of spin, increase the deltaAddDelta value.