What investment instruments should be considered when a market down turn starts and you sell your equities until your next market uptrend?
I refer to the market to the major US indices: Dow Jones, NASDAQ, S&P500
Chris Rea's comment captures it -- most people don't have working crystal balls.
Unless you are a trader, you address risk with diversification. There are several questions on this site discussing strategies to diversify your portfolio.
In many ways, the markets offer investors false choices. We have an emotional desire to react to stimuli -- but that often leads to lousy outcomes. For example, selling your NASDAQ 100 fund after its value falls is usually a bad idea.
There are really two answers, depending on your goal. You're either trying to preserve wealth, or increase wealth.
In the case of preserving wealth, undervalued blue chip stocks are always a great move. Bonus points if they have a history of increasing dividend payments. The best example I have is bank stocks - the continuously change their dividend payouts (usually to the benefit of investors), and at the same time, many have been undervalued, or over-whelped during the downturn.
Alternatively, you can let things sit, and buy high quality corporate bonds. I'm seeing (daily) offers from my brokerage's new issues list for high quality bonds paying 5.75%-7.25%. While you have to lock in your money for the period, the rate of return tends to offset lock-in worries, and you can still usually sell the shares in a secondary market, or through your broker.
A market downturn is a great opportunity to pick up valuable stocks at low prices but do note that this is quite a risky proposition and requires abundant research.
But again, as is mentioned in previous answers, the start of the downturn is evident only in hindsight, and it is always advisable to have a balanced/diversified portfolio, risking only what you can afford to lose.
In any case, indices like Dow Jones, NASDAQ, S&P500 are calculated on the basis of only a few companies that broadly represent their individual sectors.