The American Association of Individual Investors is a non profit organization who's purpose is to arm investors with the tools and knowledge to manage their finances effectively and profitably. Avoid market timing. By definition, market timing advocates an unstable risk exposure.
All sources are unanimous in their discouragement of market timing.
A portfolio's risk can be moderated by mixing stocks and debt. Stocks are claims against real assets. Bonds and cash are debt, usually promising fixed returns. Stock and debt are fundamentally different animals and, consequently, their returns tend not to follow similar patterns to each other. Consequently, combining stocks and debt moderates the portfolio's risk.