I should clarify, the clutch would be disengaged in the scenario I am describing. At say 2000 RPM, push the clutch in and lift off the throttle like you are coasting up to a stop sign. Apparently the aluminum flywheel (used by many in converting the XJS V12 to manual) plus the mass of the clutch doesn’t retain enough energy to consistently prevent stalling when the fuel injection is restarted at something like 1300 rpm. This is my understanding, @Kirbert can probably clarify with some real-world experience.
Dick Maury articulates here way better than I did: Conversion to five speed stalling issue