Fast switching of logic gates
What you're probably referring to is that as CMOS gates transition through the linear region, there is the potential that both top and bottom devices can be on at the same time.  When this happens, the gate can draw a spike of current from the supply.  Because of that, the SUPPLY pins of the IC should be decoupled to ground with one or more capacitors placed as close to the package as possible.  The capacitor is NOT connected to the input of the gates.