Shift is an iOS keyboard extension with eight keys, only. When used in its intended main mode (in landscape on an iPad), each key is intended for one of your pointers, pinkies, ring and middle fingers.
To support all alphanumeric characters of a regular keyboard, Shift uses key combinations to enter a specific letter, number or trigger a keyboard function. Common letters and the space and delete function are delegated to tapping only one key. From left to right, these are as follows: 'Space', 'a', 'e', 'i', 'o' (or 'n', depending on the set language), 'r' (respectively 's'), 'u' and 'Delete'.
Once you press on one of the eight, all remaining keys are set for specific characters or functions, themselves. So if you press and hold the first key from the left and then tap the first key from the right, you trigger the 'Next keyboard' function. If you do the reverse and press and hold the first key from the right and then tap the first key from the left, you change the language setting. Each key acts as a modifier for the other ones, which motivated the name.
In its standard mode (lower case letters), combining two keys allows for 64 (8 times 8) unique characters or functions. This left some slots to add frequent bigrams, so two letter sequences that are commonly used together. As these vary from language to language, we implemented different sets for English, German, Spanish and French.
Lower case mode is accompanied by upper case modes (for next character only or 'caps lock'), a mode for diacritics and one for numbers & more.
Shift also supports single finger input: Just press on the initial key that provides the targeted character or function and before lifting the finger swipe to key the character/function is now shown on.