Just time your button presses as they line up with the bar on the left. (nintendo-okie.com)
It's actually rather fun to activate the special, as it requires both players to time the button press together. (dualshockers.com)
A few, very simple puzzles mix up the gameplay but mostly involved easily timed button presses or just moving stuff to find an item or clear an objective. (thevrgrid.com)