[Krsqk]

I think the Trek-equivalent phased shields are called "multi-phasic." I would assume, then, that normal shields would allow a certain frequency of radiation to pass through unblocked (unique to each shield's settings, of course), while multi-phasic shields constantly change the "phase," or frequency which will pass through. This makes it much less likely that the EM radiation hitting the shields will happen to be the correct frequency.

ISTR something ST where some rogue aliens in a Bird of Prey captured Visor-Boy and programmed his visor to broadcast back to them. Then they read the settings on the engineering display to find out the frequency of the Enterprise's shields, allowing them to bypass the shields completely. That is, until someone got smart and changed the shields and blew them out of the water.