Why not just buy and hold?

Buying a broad index fund and leaving it alone is a good plan. Over long stretches it has beaten most of the people trying to be clever, and it costs almost nothing to run. If you do only that, you will not have made a mistake.

It has one property worth knowing about before you start. In the backtest the strategies on this site are measured against, the S&P 500 fell 50.8% from its peak, measured at month-end, over Dec 1970 – Dec 2016. Within those months it went lower still.

A fall like that is hard in a way the number understates, because getting back is not symmetric. Lose half your money and you need to double what is left to return to where you started — not another 50%. And the recovery takes as long as it takes; you do not get to choose the year you need the money.

That is the problem these papers set out to solve. Not earn more — they mostly do not promise that — but fall less far, by following a rule that moves out of stocks when the market broadly stops rising, and back in when it resumes. The author states the target plainly: drawdowns of less than 20%, preferably less than 15%.

You pay for that. A rule that steps out of a falling market also steps out of some rising ones, and it will sometimes sell right before a recovery. The trade you are being offered is a shallower worst case in exchange for giving up part of the best case. Whether that is a good trade for you depends on how you would actually behave in the middle of the fall — which is the part no backtest can tell you.