Last year, General Motors Co. fended off a group of activist investors with the help of an esoteric financial metric to which it had previously paid little heed.

The century-old auto maker began publicly touting the statistic, known as return on invested capital, or ROIC. It tied compensation to a 20% target and said that above a $20 billion cushion, cash it couldn’t earn that return on would be handed back to shareholders.

The...