git flow – how do I pause development on one feature to work on another

You do not need the git flow feature pause yak-Speedup command (feature pause doesn’t exist anyway). The command you want to use in place of git flow feature resume yak-Speedup is git flow feature checkout yak-Speedup; that will get you back on the yak-Speedup feature branch to continue development.

Executing git flow displays:

Try 'git flow <subcommand> help' for details.

And executing git flow feature help displays:

usage: git flow feature [list] [-v]
       git flow feature start [-F] <name> [<base>]
       git flow feature finish [-rFk] <name|nameprefix>
       git flow feature publish <name>
       git flow feature track <name>
       git flow feature diff [<name|nameprefix>]
       git flow feature rebase [-i] [<name|nameprefix>]
       git flow feature checkout [<name|nameprefix>]
       git flow feature pull <remote> [<name>]