negative resistance

Theorotical:
Convert S parameter in to Z parameters.
Add XL that is inductance to Z parameters
Again convert to S parameters.
If your S11 is >1 means you have obtained negative resistance.
You find the details about the explanations in following books
You can tally this using S11 equation. (zin-zo/zin+zo)
1) Microwave transistor amplifiers by gonzalez
2) microwave devices & circuits by Lio PHI publication

If you want to see this practically
Use simulators such as ADS
there you can directly check using tools

Here is an attempt for an obvious and evident explanation of the effect:

A negative resistance can be thought of a voltage controlled current source which drives a current back into the voltage source.
In the circuit shown this can happen when a voltage at the source pin drives a current into the FET which causes a small current through the GS capacitance and the inductance between gate and ground (for rather high frequencies).
As a result, a voltage is created across the inductance which in turn causes a drain current (normal FET operation) which goes into the voltage source at the source pin. Thus, for a certain frequency range this circuit has a negative input resistance at the source pin.

Or you can consider this structure as a three port device. When you calculate 3 port s-parameters by adding a reflection coefficient at third port ( see Solid State Radio Design-I.Bahl, Prakesh et al.) you'll see S11>1 that is equivalent to negative impedance..