I am currently reading papers discussing the Zeno Effect, which discuss how measuring a system at high frequencies can almost freeze the state of a system, or keep the system in a specific subspace of states. This can be easily seen using the projection postulate. Often the topic of decoherence comes up and how limiting the system to evolve in a specific subspace results in protection of information and prevents decoherence. Two things important for quantum computation. I understand that if the system is limited to a certain subspace probability leakage is limited too, protecting information. What I do not understand is how the the subspace is kept decoherence free. How does limiting the system to a specific subspace prevent decoherence?