For what its worth,
Hardening of the arms, fists, feet, shins, whatever is a process. Small steps over time as opposed to instant change. If you got huge bruises, most likely, you over did it a bit. It happens.
The fundamentals are circulation and massage, jau is an aid, not a magic cure.
I have been taught to massage away from the heart, so that any dead blood cells (clots) go into the hands or the feet where they are trapped in the small blood vessels, as opposed to returning to the heart or brain, where they can cause “problems” i.e. stroke, heart attack, etc.
However, a circular motion is used, with the upstroke being more gentle than the downstroke, if that makes sense. Quite often a cotton ball is used held between the thumb and forefinger.
The Jau has a warming effect, which opens the capilaries, increasing blood flow, increased blood flow brings healing, in the form of fresh blood and also releases the trapped dead blood (stasis) visible as bruises. The idea is to gently massage the tissue to move the dead blood out, and allow fresh blood in. You shouldn’t massage to the point where you cause additional injury to the bruised area.
Over time, your tissue will “scar” inside, become tougher and be able to handle more impact without “crushing” and breaking up the capillaries that then bleed and cause the visible bruise. Don’t worry, over time the scarring diminishes and you become a softie again, if you stop training.
Once you have trained up a bit, the bruising becomes less, and a 'preventative" treatment with jau after contact training releases any stasis, and no bruising becomes evident.
Now, this is not the be all and end all of herbal medicines, but a basic explanation of the basic principles from my own experience. Not “mystical” mumbo jumbo. Some jaus have additional additives which may cause various different effects, but that’s getting into specialised treatment, like iron palm, etc.
Boxers have long enjoyed the benefits of a ‘rubdown’ after sparring, using witchazel or other western ‘jau’ like Ben Gay or whatever. Its the same fundamental treatment. Heat, circulation, massage, fresh blood to promote healing.
Have a look at the diagrams on http://www.kungfuoil.com, the illustration has a large version too, just click on the image.