The offical description of Testing Pitting methodology is given HERE
Test pits are invasive, in that they dig into the archaeology and potentially destroy features and remove artifacts from their context. On the other hand they are usually only carried out on a very small scale to look at a sample of the sub-surface environment and so are an excellent way to get an idea of what lies under our feet.
A test pit is usually about 1m square and will be dug down until it reaches the 'natural' (that is the soil that does not have traces of human activity). As it progresses down one would expect it to pass through various 'contexts' which would chart the events that happened at that place. One would expect the oldest of the deposites to be at the bottom and the youngest at the top but sometimes these are disturbed by human or animal activity.
When digging test pits it is important that the progresss is monitored by the archaeologist and the people invovled have a basic knowledge of the techniques to be used. It is also very important that all finds, features and artefacts are fully recorded.
The results from the society's first training event, which was test pitting in Gayton, can be seen HERE