I'm not even saying that.
Let's say it's at £1K per ounce, for example. Do you sell if/when it drops to £950? Or is that just a fluctuation? If you do sell at £950 then you're going to be selling pretty quickly after buying. If you sell at £950 then when it goes back to £1K you realise you made a mistake and buy back then you're going to lose a lot of money.
So you have to have a more reasonable point at which to sell. Do you sell at £900? Well, what if you bought at £1K and the price went down to £950 the next day and sat there for 3 months doing nothing. Do you sell if it hits £900? Or is that a fluctuation?
So the price hits £850 and it's PANIC TIME! Everybody goes to sell. You notice about 2 or 3 hours later. You can't get through on the phone to your broker, and their internet site is bogged down. By the time you get through the price is £700 and you put your instruction in. They carry out your instruction and come back to you to say they sold it at £500.