Sharpening Mask Visualisation on iPad

I can't get the sharpening masking visualiser to work on iPad - on my iPhone I can tap the photo and toggle the slider, but on iPad it either adjusts the masking with no visualisation or shows me the 'before image'. I've read that using two fingers on the slider will bring up the visualisation, but it's not worked for me. I have a keyboard for my iPad and alt/option doesn't work either. Anyone had any joy?