Full credit to the guys at the above link for this one!!
You will need some hosepipe, some fake blood (use cornpowder, water, food colouring, corn syrup instead of powder if you can get it but I can never seem to find it in the supermarkets in the UK, EDIT: use glycerin from the baking section of a supermarket, or hair gel!!), and a manual pressure sprayer with a hand pump. I bought my pressure sprayer from Halfords for £5, you can also get them from garden stores (pesticide/plant feed sprayer). Use a clean and unused one though, you don't want to poison anyone with DDT or anything.
I have used this method myself... basically you run a garden hose (with the end of the hose blocked off) under your actor's shirt. Secure it to the body. Near the blocked-off end of the hose, make a hole in the hose wall that you point outwards away from the actor. If possible do some light scoring of the fabric that sits over the hole. Plug this hole with tissue paper or sponge, plug it quite tight but not too tight. You fill the hose with fake blood and attach your manual pressure sprayer to the other end of the hose, which will be off screen and controlled by the bloodsplosion technician. You have your bloodsplosion technician pump the bejesus out of the sprayer (50 pumps or more) to pressurise it and when they pull the trigger, the pressure shoots through the hose and sprays the blood out in a realistic and silly fake bullet hit.
Tip - make sure the end of the hose that is blocked off is blocked off tight, as tight as possible because while you intend for your directional plug to be dislodged, you do NOT want the pressure to fire the end of the hose out and drench your actor under the shirt!! Also, on the other end of the hose, make sure the hose is secure on the nozzle of the sprayer, as the pressure can fire the hose off the nozzle and spray your bloodsplosion technician in fake blood. My girlfriend and esteemed bloodsplosion tech found that out the hard way. Hehehe. You want to use as short a hose as possible to maximise the effect.
Also use a lot of blood, make it gruesome, remember you are shooting on video and things always look less impressive on the screen, so over-compensate. One more thing, be aware that the time between when you pull the pressure sprayer trigger and when the effect works is variable, it can be instantaneous and other times it might take a second while the pressure discharges before it is enough to blow the plug from the hose. This is messy too, do it outside... the food colouring can really stain a bathroom.
Edit - here is a much simpler and low-tech way, taping sandwich bags filled with blood to your body and popping the bags with your hand when you get injured (clutching your injury at impact). Again, full credit to the authors.