Bent Line Mode
From SnesLab
Bent Line Mode is one of the increase modes for controlling gain. Figure 3-7-4 of the official Super Nintendo development manual has a graph of the inverted exponential decay function that bent line mode is trying to approximate. [1] The formula that generates the graph is:
1 - ke^(-t)
but there is a typo in the manual where the exponent is missing the minus sign.