Good question!

"Planing" is a term Jan Hiene of Bicycle Quarterly coined to describe bikes he'd ridden on brevets that seemed to have a symbiotic interaction with his pedaling and upon which he was able to ride faster with no more effort. The idea was that instead of trying to achieve the modern goal of an infinitely stiff frame, an optimum frame should have a specific flexibility that worked in harmony with a individual rider's weight, ballance and pedaling motion.

The thought was that traditional, smaller diameter steel frame tubes were the most promising approach to designing a frame that would "plane".

Others have openly doubted "planing" exists.

I haven't followed the "planing" debate for around a year and I'm probably not up on the latest thinking.

Best,
tcs