  The physical processes that define the spine of the galaxy cluster X-ray
luminosity -- temperature (L-T) relation are investigated using a large
hydrodynamical simulation of the Universe. This simulation models the same
volume and phases as the Millennium Simulation and has a linear extent of 500
h^{-1} Mpc. We demonstrate that mergers typically boost a cluster along but
also slightly below the L-T relation. Due to this boost we expect that all of
the very brightest clusters will be near the peak of a merger. Objects from
near the top of the L-T relation tend to have assembled much of their mass
earlier than an average halo of similar final mass. Conversely, objects from
the bottom of the relation are often experiencing an ongoing or recent merger.
