Does a TVS diode shut the voltage off completely when the voltage is over the amount it allows?

 

No. It acts like a zener diode. It starts conducting when the voltage rises above its rated voltage. It does have some forward resistance when conducting so the voltage it reaches depends upon the amount of current it is conducting above the rated value.

 

It is a shunt regulator. It relies on having some impedance between the power source and load so that the peak current creates a voltage drop across that upstream resistance.

Read this TVS Application note

 

look here......
http://en.wikipedia.org/wiki/Transient-voltage-suppression_diode