The story
From the sounds (r, u̯, K, i̯) which triggered the change. The law is stated as a mnemonic rule because the word руки (ruki) means “hands” in Russian.
Etymology from Wiktionary, CC BY-SA 4.0.
ruki
Today it means: A sound change that took place in the satem branches of the Indo-European language family, and according to which an original *s changed to *š after the consonants *r, *k, *g, *gʰ and the semi-vowels *w (*u̯) and *y (*i̯). Full definition →