This is an incredibly effective productivity mindset that can help you better manage all of the random things that come across your plate. At first glance it seems impractical because it sounds unnecessarily disruptive, but when you do it right it actually frees up your time. Do it now.
I understand that when you’re "in the flow" and you become aware of something that can take you off task, it doesn't sound like a good idea to address it. But let’s reveal what it actually means and in doing so change the perspective on this.
What’s important to note is the following applies for when you have already been presented the stimulus. You’ve been notified of something else going on and you are debating how to respond. Of course you can eliminate the notification from the onset and prevent the disruption in the first place, but that’s a different topic.
With that in mind, when that notification or disruption does come through, do it now. As great of focus as you may have in the moment, subconsciously the notification affects your ability to focus and it demands your attention. We are evolutionarily hardwired to detect and seek to understand changes in our environment because it may present a threat. So in order to appease our threat detection systems, we can “do it now” so that we can return to or work with complete focus.
This takes two forms. If the interruption requires a small action to be addressed, something that will take a minute, do it now. Complete the action in its entirety so that it doesn't linger over your head. Alternatively, if a larger action is required of you, have a system that allows you to address it now. Write it in a notebook, or put that thought somewhere that you will revisit so that you are confident you can address it when the time is right. Then you can return to your task with focus knowing you won’t overlook the new deed that needs to get done.
In both cases, it’s as simple as doing it now. Don’t try to ignore the stimulus because it will continue to affect you more than you know.