A long time ago I wrote about my usage of the TeX input method, which is great for entering mathematical Unicode characters not present on the keyboard. Of course, I don’t have it on all the time – that could interfere with me just typing ordinary stuff (especially in AUCTeX, which I still occasionally use). My usual workflow was to press C-\ (toggle-input-method), type the character(s) I need, and press C-\ again to disable it. It turns out that in newer Emacsen, if you only need one character, you can use C-x \ (activate-transient-input-method), which lets you turn some input method to do just that. What’s even more interesting, the selected trasient input method may be different than the selected input method (i.e., the one toggled with C-\) – both C-\ and C-x \ let you change the selection with a prefix argument, of course.
This is one of the many, many examples of polishing Emacs over time. Something that is not necessary in any way, but makes editing even more pleasant and friction-free.