bootstrapping capacitor
The capacitive coupling is shown in the picture. There is a blue 3fF capacitor that couples the input of the inverter to its output.

When node a is at 0V and node b at 5V the capacitor is charged to 5V. As soon as node a rises to 5V, it pushes the a-side of the capacitor to higher voltages, while at the first moment the voltage across the capacitor stays constant, the result is that the voltage at node b rises as well. Then the capacitor starts to discharge across the parasitic diode of the PMOS and the voltage at node b also starts to decrease because the NMOS turns on.

Below a screenshot of a simulation of a simple inverter with an additonal 3fF capacitor between input and output. 



The red trace is the input, green is the output and the yellow curve is the current through the capacitor. Please note the large current spike of almost 1mA.