Hilton

He means that when leaving the car parked and in gear (like you should on a hill), leave the lever in the plane in which the centering spring wants it to be.

That way you don't place unnecessary load on the (NLA) shifter centering spring thats internal to the gearbox - hopefully extending its lifetime. Placing it in a gear where the spring is loaded will mean many more hours of fatigue on the spring.

For 86+ year cars, thats in second or third. For earlier 5-speeds, its 4th or 5th gear.