What's the basis behind this idea?
OK, I'll be the heretic.
Yes, "sell in May and go away" has worked very well in the past. Nearly all of the market's returns over several decades have come between November and May. Obviously, it does not work every year, but nothing does. The idea is to avoid a large market decline, most of which (not all) have tended to occur June - October, August and September being the months with the worst average return.
Does that mean I recommend using that strategy? No. For the last six years it has cost a lot of return to follow that strategy. That's what tends to happen with timing ideas.
I have found that aside from that strategy the only other market timing strategy that makes any sense is recognizing bubbles like the crazy tech bubble in 1999 when people were quitting their jobs to be day traders and companies with little history and no earnings came public and immediately had a value higher than huge, established companies.
Or, the housing bubble in 2007-2008 when most mortages in CA were 0%, you had pick a payment loans, more and more people were flipping houses and home prices had soared. That was not nearly as easy to see and very difficult to time and that's the problem with market timing based on bubbles - you know things are risky but they could stay that way for a couple years or more. So, as a timing strategy it doesn't work all that well in practice.
Currently we have a bond market bubble where countries are selling bonds at less than 0% interest rates. When will it end? Who knows? But, it does tell you that risk is way higher for bonds than normal, especially government bonds, and that's why my bond holdings are all fairly short term and I am looking elsewhere for low volatility income investments.
For most people, because timing is so difficult to do and because the more timing you do the worse your returns tend to be, the best strategy is to invest according to your risk tolerance and time frame and stay put.
You can also implement a stop-loss strategy of selling when you are down say 15% and either switching investments or not putting that money back to work until the market has stopped going down. You should certainly do that on individual stocks and perhaps on funds too.
Broad market index losses of 10-15% are not that rare but when they exceed 15% that has usually meant big trouble. Just don't stay out any longer than you have to because while you may avoid losses you will end up avoiding gains as well.