Servus !, you said:
ASRAAM is right, if you are in FLCH descent not following the FD the throttles will stay in idle (HOLD) until to the MCP ALT. Switching off the FD changing the AT to speed mode and everything is fine. Thats the reason why you have to switch of the FD during visual approaches.
With respect, Mostly correct. However, switching off the FDs will not automatically put the A/T into MCP SPD mode, it depends on A/T mode at FD disengagement.
If the AFDS was in LVL CHG (FLCH for the 57 crowd) prior to FD disengagement and prior to ALT capture, the A/T would remain in ARM. Pressing SPEED button on MCP restores A/T to MCP SPD mode (with FD's off, such as on a manually-flown visual approach). If the AFDS was in V/S mode at time of FD disengagement, then the A/T would remain in MCP SPD and increase thrust as required to maintain the MCP selected speed.