From A330 instructor support
NOTE:
The precomputed descent path is actually divided in successive descent segments: from TOD to the first
constrained waypoint it is an "idle segment", then geometric segments. The Idle segment assumes a given
managed speed flown with "Idle + Δ" thrust, this Δ thrust above idle gives some flexibility for the FG to keep the A/C
on the descent path in case Eng A/I is used, or winds vary. This explains THR DES on FMA. The Idle factor set on
A/C STATUS page allows to adjust Δ: e.g. if - 2 is set, the Idle segment is steeper