# Spinor helicity formalism, exact form of the spinors

I am trying to understand how to perform computations with the spinor helicity formalism, I am studying on this review http://arxiv.org/abs/1308.1697.

I have stumbled upon a problem though, in pag. 38 it is stated that

"Here we used the following convention for analytic continuation: $|-p\rangle = - |p\rangle$"

I do not understand what this means. Is there an arbitrariness in the definitions of the spinors that allows such a possibility? When I have tried to explicitly evaluate the spinors I obtained a different behaviour, the minus inside the spinor sign becomes an "i" outside of it.