For those of us engineers I suggest thinking about all pitch axis control as a matter of managing energy.
- Kinetic energy is that associated with speed
- Potential energy is that associated with altitude
The throttles and speedbrakes are used to add, maintain, or reduce energy.
The elevators are used to apportion energy between speed (kinetic) and vertical path (potential).
At all times you want to make sure you know who is minding the store with regard to both energy and how that energy is divided between speed and path.
FLCH is a way to use the elevator to control speed while the throttles / speedbrakes are fixed with extra energy going into climb or decaying energy coming from descent.
One of the keys with any transition mode of the autopilot (FLCH being one) is understanding what the next transition will be as the climb or descent clearly cannot go on indefinitely.
- When we get to a target altitude or path (such as glideslope), who takes over speed control and who takes over path control (i.e., automation modes that will engage)?
- If my target altitude or the glideslope is such that I am already above it and climbing or below it and descending, what happens next?
I'm not a pilot, so those who sit in Row 0 are the experts on the details of operating autopilot and autothrottle modes on specific airplanes, certainly not me. I just offer this as a bit of physics side thought that might bring clarity to some.