A: It’s a “shaping” or “contour” control that changes the overall tonal character of the amplifier. ISF works alongside the amp’s standard tone controls to provide tons of tonal control.
Turned counter-clockwise, ISF shifts the amp more toward an “American” sound, with somewhat scooped midrange. With the ISF turned clockwise, the sound is more “British,” with punchy midrange and top end. Many shades in between these extremes are also available using the control.