The Adjust Offset Button changes the main tuning screen so that it is replaced by three large regions. The upper half of the screen is a control where you can adjust the offset. The lower half of the screen contains two large buttons. The left button ends the offset adjustment mode and returns to normal tuning mode. The right button toggles the immediate VoiceOver announcements of the offset value while you are adjusting it. All three of these regions are large enough to locate by feel.
The offset adjusting control in the upper half of the screen does not respond to the usual VoiceOver gestures. Instead it allows immediate and direct control over the offset. As soon as you touch anywhere in the upper half of the screen, adjustments can be made. The offset is adjusted by slowly dragging your finger left or right through the top half of the screen. As you do so, the offset changes proportionally according to how far you drag your finger. If you reach the edge of the screen and want to adjust further in the same direction, you can lift your finger and start over for more adjustment. As you make these adjustments, the offset optionally is spoken. When you reach the desired offset, lift your finger straight up and tap the button in the bottom left of the screen to go back to the normal tuning mode.
If you would like to set the offset to exactly zero to return to standard pitch, just tap in the adjustment area with two fingers at once.
When you are making adjustments to the offset, normal tuning mode is still operational. You will still hear the sweeping tone that indicates sharp or flat. Therefore you can use the offset adjustment to match an existing tuning. Just play the selected note and adjust the offset until the sweeping tone slows down and stops. Keep in mind that in this scenario, if the piano is flat, the sweeping tone does sweep downward. You would raise the pitch piano string if you were tuning the piano. But you would need to lower the offset of the Montal Tuner if you want to match it to the flat piano.
The large button in the lower right of the adjustment screen toggles immediate VoiceOver announcements of the offset value on or off. When you are using the offset adjustment to match to an existing tone, you don't want to have VoiceOver telling you the offset because you are listening to the sweeping tone. But you do want VoiceOver reporting of the offset adjustments if you are adjusting with a specific offset in mind. That is why the VoiceOver reporting of these adjustments is made optional.