For years, it was the sight that stopped “showtime”: a plainclothes officer, rising to identify himself aboard a New York City subway car and greeting tip-seeking break dancers with handcuffs.

Arrests for performers onboard trains more than doubled last year. The “acrobats,” as Police Commissioner William J. Bratton called them, were held up as a signpost of disorder underground; enforcement against them, the commissioner said last year, was “soaring.”

But in recent months, police officers underground have quietly begun delivering a sharply different message on small palm cards handed to the scofflaw showmen they encounter.

“Make money,” the cards read. “Avoid arrest. Dance!”

As part of a pilot program aimed at curbing illicit performances on the rails, the de Blasio administration is urging dancers to take their act above ground, setting aside outdoor space for performances that can allow dancers to earn tips legally.