diff --git a/opm/simulators/SimulatorIncompTwophase.cpp b/opm/simulators/SimulatorIncompTwophase.cpp
index 5005c586d..0c55c8838 100644
--- a/opm/simulators/SimulatorIncompTwophase.cpp
+++ b/opm/simulators/SimulatorIncompTwophase.cpp
@@ -136,6 +136,11 @@ namespace Opm
         return pimpl_->timestep_completed_;
     }
 
+    // empty default implementation, but provided in module; it is dangerous ta have
+    // this inlined in clients from the header, because then it can't be updated!
+    void SimulatorIncompTwophase::sync () {
+    }
+
     static void reportVolumes(std::ostream &os, double satvol[2], double tot_porevol_init,
                               double tot_injected[2], double tot_produced[2],
                               double injected[2], double produced[2],
diff --git a/opm/simulators/SimulatorIncompTwophase.hpp b/opm/simulators/SimulatorIncompTwophase.hpp
index 09ea2b93a..cba56a286 100644
--- a/opm/simulators/SimulatorIncompTwophase.hpp
+++ b/opm/simulators/SimulatorIncompTwophase.hpp
@@ -110,8 +110,28 @@ namespace Opm
         /// sim.timestep_completed ().add <Foo, &Foo::bar> (f);
         /// sim.run (...);
         /// \endcode
+        ///
+        /// \note
+        /// Registered callbacks should call the sync() method before
+        /// accessing the state that was passed into the run() method.
+        ///
+        /// \see Opm::SimulatorIncompTwophase::sync
         Event& timestep_completed ();
 
+        /// Notify the simulator that a callback has an interest in reading
+        /// for reporting purposes the contents of the state argument that
+        /// was passed to the run() method. The simulator will then flush
+        /// any internal state which is currently not reflected in it.
+        ///
+        /// \note
+        /// This should only be called from within a notification which has
+        /// been setup with timestep_completed(). Avoid calling this method
+        /// outside of run().
+        ///
+        /// \see Opm::SimulatorIncompTwophase::run,
+        ///      Opm::SimulatorIncompTwophase::timestep_completed
+        void sync ();
+
     private:
         struct Impl;
         // Using shared_ptr instead of unique_ptr since unique_ptr requires complete type for Impl.