Not too long ago, Google disgruntled many power users by changing a shortcut in Chrome related to custom searches. Instead of being able to follow up a trigger word with a tap of the space bar to start the custom search, people were forced to use the tab key. Google quickly reverted the change as it became apparent that people weren't willing to adjust their muscle memory that much, but the company isn't done messing around with the shortcut just yet. It's currently experimenting with requiring double tapping space.
When enabled, double tapping the space key will enter keyword mode, as opposed to a single tap.
We spotted a flag in the latest under-development version of Chrome Canary. When enabled, it does exactly what you would expect: It changes the custom search trigger to double tapping space instead of single tapping. Since Canary is the earliest public-facing version of the under-development browser, it might take quite some time until it hits the stable release, though. And as with any of these experiments, Google could as well decide to scratch it altogether or opt for another solution.
We assume the reason why Google wants to change the space bar gesture is because it introduces potential confusion for less advanced users. For example, if someone wants to start a regular Google search query with "amazon.com" for some reason, they'd automatically be thrown into the default custom Amazon search engine — a behavior someone who isn't married to their browser might not understand. That's why Google wants to make it more complicated to trigger custom searches, and double tapping the space bar seems like a reasonable compromise between advanced users' muscle memory and regular folk's accidental searches — much better than the tab requirement for sure.