What is knee reaping? Knee reaping is a control position where the attacker’s foot crosses the hip/midline during a leg entanglement. The reason that this position is illegal is it can cause the knee to bend in an unnatural direction.
However the 3rd degree of motion which the knee can’t move is side to side in the frontal plane. The thought is that when someone is starting to reap the knee that they are attempting to force the knee in a valgus moment, therefore injury can occur. The primary constraint to frontal plane motion at the knee is the MCL (knee pushed inside), LCL (knee pushed outside) or ACL (knee pushed in and rotated). When the knee is completely straight there is stability from the bony articulation of the tibia and femur so if injury occurs it is when someone is standing and their knee is slightly bent. The ligaments are most prone to injury with 20 degrees of flexion.
Can injury occur? Yes if the defender resists this motion, there will be a great deal of force directly on the ligaments which could result in injury. However a very easy and simple solution to avoid your knee being damaged is to let your hip and foot internally rotate. Make sure to let your foot rotate as well because a planted foot may turn the force into femoral rotation, which may strain the ACL (same motion as a heel hook). When in this position if the defender lets their hip/foot rotate inward slightly then there will be no force directly on the MCL or ACL, which means injury will not occur.
It is important for jiu jitsu players to understand when their knee is in an awkward / risky position so that they do not hurt their own knee. When grappling if you feel an uncomfortable pressure on your knee it is better to let the sweep occur than fight it and injure yourself. In my opinion most injuries related to reaping are due to the defender being ignorant of the position.
As long as you understand the position you do not have to fear the reaper.
1. Logerstedt, D. S., Snyder-Mackler, L., Ritter, R. C., Axe, M. J., & Godges, J. J. (2010). Knee stability and movement coordination impairments: knee ligament sprain: clinical practice guidelines linked to the international classification of functioning, disability, and health from the Orthopaedic Section of the American Physical Therapy Association. Journal of Orthopaedic & Sports Physical Therapy, 40(4), A1-A37.
2. Neumann, D. A. Kinesiology of the Musculoskeletal System: Foundations for rehabilitation. 2010. Mosby Elsevier.