We use the framework of the Collins-Soper-Sterman (CSS) formalism to discuss transversely polarized collinear and TMD observables, where we focus on the Sivers effect in SIDIS.  
Using an improved version of the CSS framework, we are able to re-derive at leading order the well-known relation between the (TMD) Sivers function and the (collinear twist-3) Qiu-Sterman function.  
We discuss the physical interpretation of this identity in terms of the average transverse momentum of unpolarized quarks in a transversely polarized spin-1/2 target.  
We also show how it is collinear twist-3 functions that drive all collinear and TMD transverse-spin effects.