A historical demonstration in 1887 showing the weight of the central span of a bridge being transmitted to the banks through diamond shaped supports. The central "weight" is Kaichi Watanabe, one of the first Japanese engineers who came to study in the UK. Sir John Fowler and Benjamin Baker of Imperial College, who designed the Firth of Forth bridge, provide the supports.The weight is carried through compression in the wood poles and tension in the arms. The heavy supports on each end prevent the people from tipping inward, and the symmetry of the design cancels out all horizontal components of the loads. The wikipedia page for this bridge and general cantilever bridges have some explanations as well.
Quote from the discussion thread at the via.