What we are discussing is pedantic. Tutorials are made for explaining generic drills in type rating. Everyone doesn't have them. So FCOM and FCTM will obviously prevail over them. Tutorial only says Drift Down ceiling Set and pull. It nowhere says the aircraft will decelerate to GD first and then descend. It's not even given in any document. As OP DES says it is linked with energy exchange between speed and Alt. Since the thrust is locked at MCT and descent speed is low the resultant ROD will be low so the energy exchange target will take longer to reach. It may appear that it is waiting for GD but not necessarily so. If it descended 10Sec earlier may not make real difference or if it did a few knots below GD(not desirable) but won't be catastrophic. If FCTM says when ready to descend then you are ready to descend a little before GD.
Last edited by vilas; 2nd Mar 2020 at 04:35.