Kanō shihan wrote and lectured on the principle of Jū, flexibility. He termed it as Jū no Ri, the Principle of Flexibility.
What is the origin of the term and its concept? What is the correct context?
The primary saying that is used to describe the core philosophy of jūjutsu is the four-character idiomatic phrase 
柔能制剛 Jū nō sei gō
Flexibility overcomes strength 
Or, more often, and less correctly ‘Softness overcomes strength’ 
But that is only the introductory line of the Upper Strategy. The complete primary text of The Three Strategies of Huang Shingong’s Upper Strategy from near 2250 years ago reads:
軍讖曰： The “Military Prophecies” cites:
柔能制剛 Flexibility controls the strong,
弱能制強 weakness controls strength.
柔者徳也 The flexible have virtue,
剛者賊也 the unyielding have faults.
弱者人之所助 The weak attract assistance,
強者怨之所攻 the strong attract opposition.
柔有所設 At times use flexibility,
剛有所施 at times use hardness,
弱有所用 at times use weakness,
強有所加 at times add strength.
兼此四者 One using all four
而制其冝 will then prevail. 
The primary purpose of the strategy was how to deal with people. In the extended commentary it is clearly about his to deal with subordinates first. The extension is now to deal with non-subordinates, including enemies.
As one of the Seven Military Classics, this work has been studied for over 2000 years as one of the most important schools of strategic thought.
Regarding its use in describing jūdō, apparently Kanō shihan thought it was insufficient to capture his vision; therefore, he developed his own explanation of the basic principles of jūdō that went through various versions, but eventually he settled on:
The origin of Kanō’s principle is much more complex, a long tale that will be explored in the full version of The Kanō Chronicles® http://www.kanochronicles.com
 Japanese and Chinese use thousands of four character ideograms called yojijukugo in Japanese. These are used as in a wide array of situations from sayings to mnemonics to short hand for long stories or legends. Many are thousands of years old.
 柔能制剛 Jū nō sei gō is Chinese. It is rendered in Japanese as 柔よく制剛 jū yoku sei gō the quality of flexibility / softness controls hardness / softness
 The author contends that the typical translation of jū into English as ‘softness’ is neither correct nor appropriate in historical context and for the purposes of understanding jūjtsu or jūdō.
” San Lüe 三略 (Three Strategies) is divided into three parts: Shang Lüe 上略, Zhong Lüe 中略, and Xia Lüe 下略. The first two parts quote from military writings of the past, Jun Chen 軍讖 (Military Prophecies) and Jun Shi 軍勢 (Military Power) and elaborates them, while the third part is the author’s own discussion. Some attribute the work to Huang Shigong 黃石公, but in recent research, it is said that this book was written by an anonymous person between the Qin dynasty (221–206 BC) and Han dynasty (206 BC–220 AD). ”
The Governing Principles of Ancient China, Volume 2 – Based on 360 passages excerpted from the original compilation of Qunshu Zhiyao (The Compilation of Books and Writings on the Important Governing Principles), pg 508. Seri Kembangan, Malaysia: Chung Hua Cultural Education Centre, 2014.
The entire work’s name in English is usually rendered as the The Three Strategies of Huang Shigong.
The Shang Lüe 上略, Zhong Lüe 中略, and Xia Lüe 下略 are respectively the Upper Strategy, the Middle Strategy, and the Lower Strategy.
The exact date of the Military Prophecies seems unknown but appears to be around 2400 years old.
English translation by Lance Gatling, Tokyo, Japan ©2020.