Ah, why god made google...
Jobst Brandt sez:
The links are easy to disengage when clean and new, and not manually separable when used and dirty. The reason for this is apparent when considering the fit of its pins in their slots. The pins have a neck and a mushroom top, both of which are intended to bear on the side plate when in tension. To prevent inadvertent disengagement, the thin mushroom head is seated in a relief in the side plate that is more than a semicircle and thereby prevents the pin from sliding back under chain compression, manual or
otherwise.
To remove the link, it must be laterally compressed enough for the mushroom heads of both pins to rise above their retention recesses. When clean, this is possible because the chain has clearance between side plates for making lateral bends for diagonal chainlines. With use, chains take up road grit that gets into side plate lateral clearance as well as in other places. This means that the PowerLink cannot be readily compressed and disengaged manually. I find standard household pliers used diagonally across the length of the link opens it readily. Otherwise the side plates of the link must be pressed together while adjacent links are rotated to expel grit so that they can move inward to release the pins.
The reason for these retention recesses became obvious when I first tried a PowerLink with a new chain on old sprockets. When the chain skipped, as is common with worn sprockets and new chain, the PowerLink disengaged from the sudden acceleration and fell out. I retrieved the link from the road and reinstalled it with no further problems. Later I could not manually remove it when it got dirty.
Jobst Brandt
jobst.brandt@stanfordalumni.org