Callback API used for executing code at different points in the simulation. The external thread pushes input data at its own rate (typically once per game thread tick) The internal thread consumes the relevant inputs based on timestamps. For example, if a physics step is 40ms and we tick the game thread at 20ms, the callback would receive 2 inputs per callback (assuming data was pushed every game thread tick) A callback can generate one output to be consumed by the external thread. For example, you could apply a force to an object based on how close the object is to the ground. In this case the game thread may want to know how much force was applied.