The first thing I should mention here is that for any aggressive cow, calm, well-experienced handling is essential and can almost always prevent the cow from lashing out. Having said that, I have been witness to new heifers almost destroying parlours because they don't like being milked. In these instances, a kicking bar can be helpful.
When the cow approaches the parlour, try to have a few spaces open so she can almost 'choose her spot' (some cows really don't like to go into some spaces and the feat of getting them in there is a whole different job in itself - so just let her have free roam of where is being milked).
Once she is into her space, feed her and put a rope or chain round the back of her to stop her from walking backwards. Gently pat her on the hind quarter to let her know you are there, take the kicking bar and firstly use the bottom hook to hook under the top of her leg, just above the udder, where there is a flaccid-like ligament hanging down. Once hooked under there, swiftly but calmly push upwards and hook the top of the bar round the top of her spine, with a slight angle towards her tailhead, so that it is fully pressing on the part of her leg that is stopping her from kicking.