...nnnooope... convection still works. Water in the block and its connecting tubes expands. This displaces cooler, contracting water in radiator which is being pushed into the res. Water from the res, meanwhile, because of gravity, pushes down into the pump and upwards to the block again. This gives the aforementioned expanding warm water from the block a firm foothold to push off against, into the rad where it displaces the cooler water...
If this is how it works, you should find that things will go ass-up if you lower the position of the res (so perhaps you don't want to do that...
)