Internally, a NeuronGroup with refractoriness has a boolean variable not_refractory added to the equations, and this is used to implement the refractoriness behaviour. Specifically, the threshold condition is replaced by threshold and not_refractory and differential equations that are marked as (unless refractory) are multiplied by int(not_refractory) (so that they have the value 0 when the neuron is refractory).
This not_refractory variable is also available to the user to define more sophisticated refractoriness behaviour. For example, the following code updates the w variable with a different time constant during refractoriness:
G = NeuronGroup(N, '''dv/dt = -(v + w)/ tau_v : 1 (unless refractory) dw/dt = (-w / tau_active)*int(not_refractory) + (-w / tau_ref)*(1 - int(not_refractory)) : 1''', threshold='v > 1', reset='v=0; w+=0.1', refractory=2*ms)