witching, as far as i am aware, is when the derivatives markets expire.
eg, when the futures markets, stop futures, options etc reach maturity.
when they do, eg last week or so of each quarter, you tend to see exceptional volatility as people are shifitng out of positions and rolling into the next quarter's contracts and so on.
its generally wise to keep clear of the markets on expiry day, as things can get more than a little hairy.
eg last week (or whenever it was, im losing my mind) the June FTSE futures expired with a final 70 point spike in a few minutes that wasnt reflected on the cash index.
there are probably some more detailed explanations on this site somewhere.. a search should yield results.
fc