TURBO OverDrive

From the manual:

The BOSS TURBO OverDrive OD-2 can give strong distortion effect without spoiling your delicate guitar playing technique. The TURBO ON/OFF Switch allows you to select either of the two types of the effects. That is, usual delicate overdrive sound when TURBO is OFF, and powerful overdrive sound nearly as strong as distortion when ON. The TURBO ON mode also allows a natural feedback effect with a little howling.

https://archive.org/details/BOSS_OD-2_Manual/