I agree, the concept of "falling knife" is rather strange. Especially for new users. But in fact, everything is simple. The term falling knife refers to a sharp drop in the price of something. If we consider the topic of mining, then in this case we are talking about a cryptocurrency that instantly lost its value. The reasons for the drop of the knife can even be bad news or changes in market sentiment. There is a definition of falling knife
https://minery.io/glossary/falling-knife/ . Sometimes traders decide to catch a falling knife while stalking their targets. But this is very risky and sometimes has a negative result.