“In addition to the longhand methods, computus manuscripts often included mnemonics and lists for easy reference. One example of this is the medieval mnemonic “post epi pri pri pri di di di pascha fi”, which reminds the reader that in a given year Easter is the third Sunday after the third new moon after Epiphany, a system which works in all but two very specific cases.
A common variation on the standard computus manuscript was the so-called computus manualis which explained how to use the hand as a way to calculate by counting along the fingers and around the palm, and allocating each joint of each finger to the months, dominical letters, and other important pieces of information.
Thomas F. Glick, Steven Livesey, Faith Wallis, Medieval Science, Technology, and Medicine: An Encyclopedia, Routledge (2014) p.111.
Concerning the “Jewish Passover” computations which are mentioned by Michael of Rhodes in connection with one of his mnemonic hands, there is a passage in Stern and Burnett which reads:
“In elucidating the difference, the author makes reference to the principle of counting the years of the cycle on the phalanges of one’s fingers, starting with the tip of the thumb for year 1 and finding the 19th year on the tip of one’s little or auricular finger. The author explicitly states that this is the principle ‘taught in the computus manualis’, which is probably simply meant to denote the general practice of finger-reckoning, but which may perhaps be a reference to a short metrical treatise of this name ascribed to a certain Johannes of Polonia, which is often found in the same codices as the Computus Iudaicus“..
and to show just how long the ‘computus manualis’ remained in vogue, here’s an eighteenth-century version (click to enlarge):