How does bone marrow acts like a valve?
First of all, though bone marrow is not considered a part of lymphatic system because it is not connected to lymph vessels, yet it is the site of production of both B- and T-lymphocytes and is thus considered as primary lymphoid organ1. So, it is closely related to lymphatic system.

On doing some research, I can conclude that bone marrow itself actually does not perform the role of a valve. It is the lymph vessels itself which perform the valve-like function. See this2:


  As lymph capillaries carry lymph away from the tissue spaces, they merge to form larger and larger vessels. These larger lymph vessels resemble veins, but their walls are thinner and they have more one-way valves to prevent lymph from flowing backward. Whereas the cardiovascular system has a pump (the heart) to move fluid (blood) through the system, the lymphatic system does not. It relies on the contraction of muscles to move lymph throughout the body. The larger lymph vessels have a layer of smooth muscle in their walls that contracts rhythmically to "pump" lymph along. The contraction of skeletal muscles, brought about by simple body movement, and the mechanics of breathing also help to move lymph on its way.




So, I'd consider that paragraph as one of the few places where Wikipedia has given incorrect fact.