Help me please again, with TRC Lesson 3.

Captain Randazzo,Before completion of the TRC Lesson 3, I have not yet understood only your instruction "set your speed bug to 250 knots and press the SPD button to instruct the auto throttle to maintain this speed once you reach it." on the TRC 3 Maneuvering page 3-4, because in this sequence the thrust mode is in TRH REF with pressing the TOGA button after the engines have stabilized at around 70% of N1.The manual AUTOMATIC FLIGHTMANAGEMENT SYSTEMS page 8-12 describes in SPD Switch: "cannnot be engaged while the auto throttle is in THR REF mode."How I should read this instruction?Sincerely,Kan-ichiro Fushihara