TL;DR - The pilots failed to engage in a 'positive exchange of flight controls' procedure and ended up providing opposite control inputs at the same time, so it

TL;DR - The pilots failed to engage in a 'positive exchange of flight controls' procedure and ended up providing opposite control inputs at the same time, so it felt to both of them like the controls were unresponsive / jammed. This is a well understood and generic problem in dual-control aviation - see https://www.faasafety.gov/files/helpcontent/Courses/ALC-36/content/index.html#/lessons/FMa2I3Uu-4AfYzF_t6gCoAvssrE-cqkE https://www.faasafety.gov/files/helpcontent/Courses/ALC-36/c... for example. The standard exchange of flight control procedure is designed to create a common understanding between the pilots as to who is in control of the plane. Interestingly it's similar to a TCP three-way handshake: Pilot A: You have control of the plane. Pilot B: I have control of the plane. Pilot A: Acknowledge you have control of the plane.