Both FDs OFF:
‐ The FD bars disappear from both PFDs.
‐ If no AP was engaged, lateral and vertical modes disengage. The A/THR, if active, automatically
reverts to (or remains in) SPEED/MACH mode.
‐ If one AP was engaged when FDs are switched OFF, this AP remains engaged in the active
modes but the FDs are no longer displayed.
From the FCOM's here:
DSC-22_30-20 P 7/10
I interpret that to mean that if you switch the F/D's off but leave the A/P engaged, there is still an interaction between what you do on the FCU and the managed guidance. Therefore, if you pull for open descent OR negative V/S, the DES mode will engage.
If the A/P is not engaged AND the F/D's are off then it will stay in whatever phase you were in i.e. CRZ.
Therefore to keep the predictions you would need to already be in the DES phase before switching the F/D's off or switch the F/D's back on, pull for open descent and then switch the F/D's back off.
More easily, just use your mental maths for descent planning if the F/D's are off.
FIS.