How to get old Dock Exposé on Mountain Lion?

This would make scrolling up (or dragging up with two fingers) over an icon show app Exposé:

defaults write com.apple.dock scroll-to-open -bool true
osascript -e 'quit app "Dock"'

Thankfully, this feature isn't completely gone. It has just, for some reason, been whisked away into the depths of less-than-obvious multi-touch gestures in OS X Lion and newer. You now have to double-tap an app icon on the Dock with two fingers to activate Dock Exposé.

I'm not sure if this gesture ever existed on OS X Snow Leopard.

You can also choose a key sequence for that using the Mission Control Preference Pane. The default is ⌃↓.