Generally a "good" sign when even the most strong and resilient coins are selling off and are part of the bigger losers today. ZEC & HYPE for example. At least at that point you know you had a proper flush and scare in the market when the favorite coins are selling off as well. No place left to hide at that point. Definitely doesn't mean a bottom gets set immediately either, but it is something you do tend to see at the start and near the end of bigger market corrections.