Can anyone help? I just opened an internet account with TDWaterhouse and was wondering how the stop loss facility works.

ticker - There are widely differing stop-loss mechanisms between differing brokers.
You need to get exact details of the functionality from your own broker, to be sure you are happy to use their facility.

e.g. With comdirect the stoploss works on bid alone, so a cheeky extremely low bid on a SETS stock at market opening, may sell your stock automatically for a very poor price. You will also find that, if there is some news that causes your stock price to drop rapidly, the RSPs drop the online trade quantity, so your order may still fail to trigger due to being over the new lower online trade size.

Other brokers have more complicated algorithms that check the spread as well, to avoid this situation.

Personally, I don't use stops but your decision will depend on how much time you are able to spend monitoring yopur portfolio.