Don't know about DFDR (except it's a FLA* I've not seen before)
Needle split on Tacho just shows one bit's going but the other isn't (or is but at different speed)
As in..
Autorotation - rotor speed higher than engine ('cos it's rolled back or broke)
Engine started but clutch not engaged, so rotor not turning
Perhaps one freewheel on a twin slipping (would give slight upward split of slipping eng needle - not good, gearbox either poorly or pilot being very unkind!)
Hope it helps
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Ground tested, no fault found