Loupe deck- the alt key highlight function for blacks and whites

[rob-nikon]

At the photography show I was shown a way to bring up the highlights similar to the way the alt key highlights the screen when setting the blacks and whites. For the life of me I can't find how you do this on the loupe deck. If i use the alt key on the keyboard the loupe deck black and white dials don't work. any ideas how you get the alt key highlight function working?