How can I search through fish completions?

I've noticed that sometimes, when there are multiple completion options, I get something like an input field, beneath the command line, where I can type and filter the completions. I'm not sure how I triggered it. Is there a key combination that I can press so that I have it available on demand?

Solution

To enter search mode directly, you can press shift-tab. You can also press tab until the highlight is in the pager, and then just start typing: that triggers searching too.

Update March 2020: This changed in fish 3.0 here. There's now two ways to reveal the completion search:

Press ctrl s to toggle the search field while the pager is active
Press shift tab instead of tab to trigger completions with the search field shown immediately

These correspond to pager-toggle-search and complete-and-search key bindings.