Why can't the distal interphalangeal (DIP) joint be actively extended if the proximal interphalangeal (PIP) joint is fully flexed?

What will be an ideal response?

ANS:
When the PIP is in full flexion (produced passively, or actively by the flexor digitorum superficialis [FDS]), the oblique retinacular ligament is slack at the PIP and, therefore, slack at the lateral band. Without the tension on the lateral band produced by the ORL, the terminal tendon of the extensor mechanism is also slack. The flexion of the PIP also creates tension on the central tendon of the extensor mechanism, pulling the hood distally. The distal migration of the hood also releases tension on the lateral bands and terminal tendon. Without tension on the terminal tendon, DIP extension cannot be actively maintained.