After you turn on the 'Undo Send' option in Gmail Labs under your Settings section (but reload Gmail first), Google will mercifully give you 30 seconds to change your mind about sending your last email. Just click the 'Undo' button, and Gmail will come to the rescue.
Is there anyone, who hasn't made a sacrilegious typo even children in primary school wouldn't dream of making, who wrote a couple of really bad words in anger, but was later sorry, or who just pressed 'send' and that email flew to the totally wrong person and left you feeling utterly embarassed? We've all been there and it wasn't pretty.
Perhaps there were some who realized this, but the majority of Gmail users hadn't had the faintest idea the 'unsend' option has been around for about six years! It was, however, cleverly hidden away in the Labs section and set for (very long) testing.
'Undo send', as its name suggests, offers a short, limited period of time during which you can change your mind about sending your email. You are able to undo what you did for between 5 and up to 30 seconds, but it really depends on what period you've set yourself in your settings. 'Undo send' can be activated in the web-based version of Gmail only, since this option is not yet available for the Gmail mobile app.
What you need to do is click on your 'Settings' and look for 'General settings'. Check-mark 'Enable Undo Send' and select the number of seconds you wish to have at your disposal to reconsider sending your email. You can opt for 5, 10, 20 or 30 seconds, but 5 is, honestly, hardly enough. There, you are all set to go and try it out!
For those who switched the 'Undo send' option while it was still in its experimental phase, nothing will change and the option will remain activated.