Once it's enabled, a little notification bubble with a link to "Undo" appears at the top of the Gmail window after you send a message. Again: Easy!
Enabling this particular Lab experiment takes a bit of extra work, however, as the feature straddles the world between Labs and a full-fledged Gmail feature in Gmail. Once you've enabled "Undo Send" in Labs, open the Settings options once again. Scroll down under the General tab until you see "Undo Send."
Next, click the check box next to "Enable Undo Send" if it isn't enabled already. Click the drop-down menu underneath it and select the amount of time you'd like for your undo send grace period. Finally, scroll down to the bottom of the settings page and click the "Save Changes" button.
Green robot is a really simple, but handy experiment that lets you know when your chat contacts are on on a mobile device, cluing you in to expect shorter or delayed responses. This experiment displays a little robot icon next to your contact's name in the Gmail chat window if they're on a mobile device.
Note that this lab only works if you are using Gmail's old chat interface rather than the new Hangouts format. Google has yet to force all Gmail users over to Hangouts, so this lab still works—at least for now. If you'd like to switch back to Gmail chat from Hangouts (PC only), click on your name at the top of the Hangouts/chat window in Gmail. At the bottom of the drop down menu that appears, click Revert to old chat.
Once you've got these Labs working, explore the rest of the experiments tab to see what else you'd like to try. Other big favorites with Labs fans include Flickr and Picasa previews, Multiple Inboxes, Custom Keyboard Shortcuts, Right-side Chat, and Canned Responses. Check them out!
Sign up for CIO Asia eNewsletters.