On many portable speakers like the JBL Flip 4, this is activated by pressing and holding the and Volume Down (-) button simultaneously for about 10 seconds.

Based on common audio hardware features (particularly with JBL speakers), the (often activated via a hidden service command) is a special setting designed for testing and enhanced bass performance, rather than daily use. Features of Low-Frequency Mode ("Service Mode")

It disables the standard Digital Signal Processing (DSP), meaning the speaker skips default sound optimizations to create a raw, loud, low-end sound.