A single case-insensitive sequence of keys, containing:
zero or more modifiers (e.g. Ctrl, Alt, Cmd, Command, Opt, etc.)
one non-modifier key (e.g. u, 4, ., Space, Tab, Pause, F3). When key names have multiple words, quote and use spaces (e.g. "Print Screen").
[On macOS] alternatively, utf-8 characters for mac keys (e.g. ⌘, ⌥, ⇧, ⌃)
A plus (+) sign separating each alphabetic word (but not key symbols) from the next
The empty string (""), to disable the hotkey.
Caveats:
There are some limitations on what keys and combinations are supported, depending on multiple factors such as keyboard, mapping, and OS (e.g. AltGr, numpad, or media keys may or may not work; shift+enter is rejected).
Some combinations may be grabbed by the OS before they reach multipass (e.g. meta+a may open the Applications, ctrl+alt+f3 may move ttys).
I came to this thread via Google and would like to recommend the same thing as @fpbhb did.
Multipass is a god send on the Mac, but on OSX, the default hot key clashes with the View Source shortcut on all the browsers — not just Chrome, but on Safari and Firefox also.
As a developer, we use these shortcuts quite often when developing web apps. I have now completely disabled it but I’d recommend changing it to something else. Since the keyboard shortcuts for browsers can be pretty crowded, and assuming that you possibly wanted to use the U key as mnemonic for Ubuntu, you can consider using:
⌃⌥⌘U (Ctrl + Option + Cmd + U) as default instead, since using Ctrl in combination with Cmd is highly uncommon with Mac software — typically, keyboard shortcuts in Mac with the Cmd modifier would translate to the Ctrl modifier in Windows. Having all three modifiers also have great usability because it’s easy to just hold these three modifiers down which are right next to each other.
I personally use these three mods plus keys for many system-wide things specifically for this reason.