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Full extension → Taut anterior cruciate → No further symmetrical extension → Medial femoral condyle moves back – lateral condyle moves forward → Femur internally rotates on tibia on axis of anterior cruciate ligament → Medial/lateral collateral and oblique popliteal ligaments tighten → Tensor fascia lata and gluteus maximus tighten iliotibial tract → Knee hyperextends and locks
Locking occurs as a result of internal rotation of femur during last stage of extension and is produced by quadriceps femoris. It allows knee to remain in the position of full extension as in standing without much muscular effort.
Further reading: https://epomedicine.com/medical-students/screw-home-mechanism/
