
The horizontal tear is a shearing injury. It is revealed by probing of the meniscus.

Probing a trimmed horizontal cleavage tear during an arthroscopy
A horizontal cleavage tear is a horizontal shearing of the fibres within the interior of the knee meniscus, effectively cleaving it into two parts, like a clam. Page updated August 2024 by Dr Sheila Strover (Clinical Editor)

The horizontal tear is a shearing injury. It is revealed by probing of the meniscus.

Probing a trimmed horizontal cleavage tear during an arthroscopy
However, more often the meniscus is already degenerative from previous trauma, and twisting just makes this worse.
Peer-reviewed papersQuote from peer-reviewed paper:
Citation:Raj MA, Bubnis MA. Knee Meniscal Tears. [Updated 2023 Jul 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK431067/
Surgeons will often choose a partial meniscectomy or even a total meniscectomy, preserving what they can of the rim. Repair might be attempted after roughening up the inside of the tear, pushing a lump of blood clot (from the patient) into the defect, and suturing it in place.
Peer-reviewed papersQuote from peer-reviewed paper:
Citation: Woodmass JM, Johnson JD, Wu IT, Saris DBF, Stuart MJ, Krych AJ. Horizontal Cleavage Meniscus Tear Treated With All-inside Circumferential Compression Stitches. Arthrosc Tech. 2017 Aug 14;6(4):e1329-e1333. doi: 10.1016/j.eats.2017.05.016. PMID: 29354436; PMCID: PMC5622296.
Quote from peer-reviewed paper:
Citation: Mameri ES, Verdejo FG, Dasari SP, Kerzner B, Shah S, Kasson LB, Khan ZA, Fortier LM, Damodar D, Gursoy S, Chahla J. Repair of a Horizontal Cleavage Tear of the Lateral Meniscus with Circumferential Compression Stitches and Marrow Venting Augmentation. Arthrosc Tech. 2022 Oct 20;11(11):e1937-e1944. doi: 10.1016/j.eats.2022.07.008. PMID: 36457397; PMCID: PMC9705603.

A valve-like action can force the fluid out with each step.
Peer-reviewed papersQuote from peer-reviewed paper:
Citation: Haratian A, Bolia IK, Hasan LK, Fathi A, Solaru S, Homere A, Petrigliano FA, Weber AE. Arthroscopic Management of Meniscal Cysts: A Systematic Review. Orthop Res Rev. 2021 Sep 17;13:123-139. doi: 10.2147/ORR.S321893. PMID: 34557043; PMCID: PMC8455512.
The surgeon is probing the horizontal tear of the meniscus to see how deep it is.
Video courtesy of Dr Ram Venkatesh
Doctor's report causes confusion regarding a cleavage tear.
Brief discussion about meniscal cyst being associated with horizontal cleavage tears.