The Spiral tool is a SOP node mainly implemented in the VOP context. Working in 2 distinct modes, firstly as a Spiral geometry generator, similar in functionality to the Line SOP as it effectively extends the Line SOP. Secondly and more interestingly the Spiral SOP is build upon input geometry, eg. If the input is a line or curve the spiral will be defined by that geometry and the spiral can orient along that line/curve.
In the below screenshots I've setup 2 nulls which define the start and end points to a 4th order nurbs curve, to demonstrate the ability of the Spiral SOP.
By changing the start and end points positions, the spiral seamlessly windes around the underlying curve. Another handy and powerful use is chaining, more complex spirals can be created by this method, this can be seen in the last screenshot were I've created a coiled cable similar to an old phone cable.