He said writers often think of their writing sessions as being part of something longer, something big. They can be setting themselves up for failure. Not completing whatever big thing they had in mind is...well, at least some form of failure.
But with postcard writing, you're thinking small. Instead of one big success, assuming you get to the end of that big thing I mentioned in the last paragraph, you can have many small successes. And perhaps they can all be pieced together into something big.
Small Units Of Time Vs. Small Pieces Of Work
I've often written here about taking advantage of small 90-minute, 45-minute, and even 20-minute units of time when we just don't have a week, a weekend, or even a full-day to work.
But I've said less, if anything at all, about breaking big work into small work. Not just chapters and scenes, but really small pieces. Like dialogue and descriptions and transitions. Try making those part of your planning, especially during periods when writing time is hard to come by, and see how much you're able to get done.
I'm interested in postcard writing and flash for the sake of those forms. But thinking about writing small could mean getting to something big, too.