Compares price change between current symbol and other one (eg. BTC vs S&P500). It calculates price change on each bar (from high to low or from open to close) and compares with price change of equivalent bar from the other source.
Example Current symbol
open = 10 USD
close = 7 USD
change = -3 USD
% change = -30%
Second symbol
open = 3 USD
close = 4 USD
change = +1 USD
% change = +33%
Performance of price change = (-30) - (+30) = -63 // It means that current source has weaker performance right now