It is all miners' outflow divided by its one-year moving average. If miners withdraw an unusually large amount of Bitcoins compared to the past year, the MPI increases, and the Bitcoin price is likely to be bearish. If you draw lines when MPI is higher than 2.5, it often matches local and global highs. On the other hand, if the MPI is low, miners hold Bitcoin, meaning it's likely to be bullish. They were HODLers in 2018 and 2020 great sell-offs though.