P.28 of the Montage Supplementary doc (p.153 of Montage M Operations doc) have wording like this:

Operation [PERFORMANCE]  [Play/Rec]  [Pattern]  [I] (Record) button  [R] (Play) button

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Hold [SHIFT] + key to erase
You can delete specific note events by pressing the [SHIFT] button and playing the keys corresponding to the notes you want to
delete.

The 'Erase' button on the display only appears when you use the mouse trail shown. That is, you need to be in record mode and press the play button.

Of course you need to make sure you have selected 'Record Type->Overdub' instead of the default 'Replace' or you will wipe out your recording.

Question: How do you make sure you erase the right notes?

That 'Hold [SHIFT] + key to erase doesn't appear on the screen until you press 'PLAY'.

But as soon as you press play the pattern starts playing.

You have to press 'SHIFT + key' at exactly the right moment in order to erase the note. But if you wait until you hear the note it is too late.

Has anyone used this 'feature'? If so, how do you use it properly? 

I am just trying to learn how to use this functionality - I don't currently have a 'problem' to solve.

But I can't figure out how to sync the 'SHIFT + key' with the actual playback of the pattern.