The rear head breather hose on an SSW+ engine performs the same function as the breather bolts on an Evo engine; that is, venting excess crankcase pressure to the outside, without allowing outside air back in. Both engines use a check valve to perform this function.
Evo engines use a pair of umbrella valves located inside the rocker boxes, while SSW+ engines use a one-way check valve, S&S Part #50-8122. It is located outside the engine, placed in the aforementioned rear head breather hose. The one way check valve must be used with the rear head breather hose, no exceptions. (By the way, SSW+ rocker boxes will have provision for Evo style venting, but it is not functional).