Why did RCR borrow Hendrick engine?

Richard Childress borrowed an engine from the Rick Hendrick team for a few laps at Daytona 500 testing, to see if the problem was engine or aerodynamics. The Hendrick engine provided more speed. Winston-Salem Journal

What appeared to be a simple engine change caused quite a stir in the garage at Daytona. Crew members installed a Hendrick Motorsports engine into the #29 Chevrolet owned by Richard Childress Racing, which has its own engine shop. Childress said the experiment was initiated by General Motors, which supports both teams. Drivers using Hendrick engines have dominated preseason testing, and Childress' teams have struggled. Scott Riggs and Nemechek, using leased Hendrick engines, have been fastest on Tuesday and Wednesday of this week. The #29, driven this week by Kerry Earnhardt, who is filling in for Kevin Harvick, was 38th fastest Wednesday. The switch was not completed in time for Earnhardt to run a lap with the Hendrick engine. Atlanta Journal-Constitution