What's the microwave slow-wave effect?

Anyone knows the microwave slow-wave effect? I can not find any reference on this. What is the effect on resonator? Thanks.

hi

suppose that i have a 2-element microstrip patch array that is resonating at 6GHz .
if i use a DGS between the elements to benefit the bandstop characteristics , the center frequency falls to 5.9 GHz . this is called SLOW WAVE effect .