Slew rate refers to the rate of change of the voltage level. Clocking signals are typically square waves, this is common to have a faster slew rate compared to other types of signals. The quick change in voltage level causes a current surge in the circuit, creating an EMI spike. To mitigate this, devices such as the CDC6C have slew rate control (also known as adjustable rise or fall time). Decreasing the slew rate reduces the current surge, resulting is a lower powered EMI spike.