I think you should cool your jets and read the thread. He has a Gravely ZT HD 48, which has the exact same manual as my Gravely ZT HD 52.You mean YOUR manual for YOUR mower says to engage at low speeds.
However we can only assume Weed Whisperer has a Gravely because he listed it in his profle.
But which model is currently unknown so what speed is currently unknown.
What is universal however is you should engage the blades with the deck set to whichever height has the deck & drive pulleys as close to in line as possible.