Basically you'd get your plaster on the wall as per usual, let it firm up slightly - you need a nice clean bucket of water and a nice clean sponge float, wet up your sponge float and run it firstly along your angles then, in a figure of 8 pattern, over the entire wall ensuing that keep the float wet and relatively clean so as to not drag the plaster, then let the plaster firm up again slightly and do your finish trowel as normal - its a brilliant technique for highlighting low spots/indescrepancies etc.