The Dream '65 Reverb Amplifier features two separate control layers. The Amp layer provides access to all of the main controls. When the middle toggle is in the up position, this will access the ALT layer.
The ALT layer provides access to the amp's vibrato controls. While using the ALT layer, the treble control adjusts the intensity of the vibrato while the boost control adjusts the speed.
Once the vibrato has been engaged, switching to the Amp layer will keep the effected sound. To disengage the vibrato, switch to the ALT layer and turn the intensity all the way down. Lastly, switch back to the Amp layer to edit the main controls.
To learn more about Dream '65 Reverb Amplifier, view the manual here.