The Ernie Ball volume pedal is the most famous of all volume pedals. The "Junior" version purely relates to its size. Swell!

"The Ernie Ball VP Jr 250k provides all the same great features as our original volume pedal while reducing your footprint by 22%. The VP 250k Jr is perfectly voiced for passive signals and features a compact, rugged design consisting of aircraft grade aluminum housing that is virtually indestructible. Directly behind the input jack under the footplate is a micro taper switch which provides the user two distinct volume swell rates."