For example, when you have two keys in a roll pattern that normally require using the same finger twice, such as 'yu' (on azerty/qwerty) followed by a character on the left hand. When the index is on the 'y', the hand has moved, and using the middle finger to type the 'u' make more sense, for me, than using the index a second time.
There are almost no words in English that include “yu” in them, so I doubt this comes up all that often. I don’t too often discuss yurts or yuccas, picayune things, or bradyuria. Speaking for myself, I type both “y” and “u” with the same right index finger in such situations. (Though my typing speed only tops out at about 100 WPM, so perhaps I can’t be considered “proficient”.)
The full list from:
egrep -i ".*yu.*" /usr/share/dict/words
Ahantchuyuk, Astrocaryum, ayu, Ayubite, Ayyubid, Banyuls, Brachyura, brachyural, brachyuran, brachyuranic, brachyure, brachyurous, Brachyurus, bradyuria, Bryum, Cambyuskan, Cayubaba, Cayubaban, Cayuga, Cayugan, Cayuse, Cayuvava, Centrosoyus, Chinchasuyu, colipyuria, colyum, colyumist, coyure, dacryuria, dasyure, Dasyuridae, dasyurine, dasyuroid, Dasyurus, Dasyus, embryulcia, embryulcus, Fayumic, gavyuti, guayule, Hiroyuki, Hyrachyus, Jianyun, Kikuyu, Kyu, Kyung, Kyurin, Kyurinish, Leucobryum, Liyuan, muyusa, oxyuriasis, oxyuricide, Oxyuridae, oxyurous, pacouryuva, picayune, picayunish, picayunishly, picayunishness, polyuresis, polyuria, polyuric, pyuria, Ryukyu, sarcodictyum, Seiyuhonto, Seiyukai, skyugle, Stachyuraceae, stachyuraceous, Stachyurus, Syun, Takayuki, Teruyuki, Tuyuneiri, Vayu, Wanyakyusa, Yuan, yuan, Yuapin, yuca, Yucatec, Yucatecan, Yucateco, Yucca, yucca, Yuchi, yuck, yuckel, yucker, yuckle, yucky, Yuechi, yuft, Yuga, yugada, Yugoslav, Yugoslavian, Yugoslavic, yuh, Yuit, Yukaghir, Yuki, Yukian, yukkel, yulan, yule, yuleblock, yuletide, Yuma, Yuman, yummy, Yun, Yunca, Yuncan, yungan, Yunnanese, Yurak, Yurok, yurt, yurta, Yurucare, Yurucarean, Yurucari, Yurujure, Yuruk, Yuruna, Yurupary, yus, yusdrum, Yustaga, yutu, yuzlik, yuzluk
Even for more common such combinations, like “tr” and “mn”, I find that I type both letters with the same finger. Really what those pairs highlight though is that QWERTY is a terrible layout – “gr”, “fr”, “ft”, “br”, “mu”, “nu”, “my”, “ny”, “de”, “ws”, “sw”, “hu”, “lo”. “ki”, &c. &c. should all (to the extent possible) be moved onto different fingers. It’s entirely possible to create a layout where pairs of letters typed on the same finger are extremely rare.