I am unsure if the logic is different on more modern boeing types, but on the 747/757/767 FLCH is very similar to an open clb/des.
There are a few subtleties however.
If engaging FLCH more than about 2000' away from your target alt, the A/C will give you full thrust in climb and idle in descent.
As aerocat mentioned, the A/THR will go into HOLD mode after 5/10 seconds in the descent ONLY. You can then move the trust levers to whatever position you like and the aircraft will maintain speed by adjusting the pitch.
If engaging FLCH less than about 2000' away from your altitude target, the A/C will give you sufficient thrust to reach the commended alt in 1 to 2 minutes.
The 2000' ish alt difference and the time the A/C allows itself to reach the target altitude is from memory as this information as been deleted from my manuals, so please take those figures with a grain of salt.
But basically, big altitude change, FLCH=OPEN CLB/DES (but you can move the thrust levers in the descent and the a/c will keep them wherever you left them)
Small altitude change, FLCH=V/S, you just don't know what the a/c is actually targeting.
It should be similar logic on the 777/787, not sure about the archaic 737
. Please correct if I am wrong