Zener diodes for JTAG interface
These zener diodes are called a TVS or transient voltage suppressor.
They have various &quot;trigger&quot; voltages where they start to conduct, and can handle large currents for a very short time.
Their trigger voltage is an estimation. Each device will vary slightly.
When they do trigger, the voltage may still rise somewhat.  The more power behind the triggering event, the higher this voltage may be.

For example, a 3.3v &quot;stand-off&quot; device is designed to operate at 3.3v, so should not affect the circuit at all.
The trigger voltage for this hypothetical device may be 4.5v.  At 4.5v, 1mA could flow through the TVS.  This increases (non-linearly) with current:

5v might cause 10mA to flow through that TVS
10v might cause 1A to flow
15v might cause 10A to flow, etc.


Research TVS's and find a device with an adequate stand-off voltage.  Understand that the voltage downstream of the TVS will exceed the trigger voltage when activated, and may grossly exceed this for a very short time (under worst possible scenarios.)
A spark from a finger is a very high voltage, but at a very low current. A TVS is able to quench such a spark in a very short time, but the voltage on that pin will rise to above the trigger voltage.