A spunding valve is an adjustable pressure relief valve with a gauge attached to show internal vessel pressure.
They are most commonly used to do keg to keg transfers of carbonated liquid under pressure.
To do this attach your regulator to the full vessels gas inlet post (fermentasaurus for instance) then join the 2 liquid post together with 2 liquid disconnects and a length of beer line.
Attach the spunding valve with a gas disconnect to the empty vessel (mini keg for example).
Set the pressure on the regulator to 12-15psi and then unwind the spunding valve until gas begins to vent (unwind the ring pull at the end to adjust pressure release point). Set pressure to 8-10psi on it and the liquid will transfer across under pressure without losing any carbonation.
It can also be used to set a release point for pressure in the fermentasaurus, this is very useful as you can set it to approx 12 psi (depending on fermentation temperature) and once finished the beer will already be carbonated (you will need to either chill the fermenter to allow the liquid to hold carbonation while you bottle or use the method outlined above to tranfer under pressure to stop it losing the carbonation while warm though.