Andrews explained that to pull off that flying sequence, she actually performed the stunt herself while wearing a painful harness. But the supposedly load-bearing wires she was attached to high above the stage didn't end up working as planned.
"There was a very dangerous day right at the end of filming when I was in this excruciatingly painful harness," Andrews told Colbert, as she pointed upward. "And I was hanging around up there for the longest time with the umbrella."
After floating in the sky for quite some time, Andrews sensed the wires giving out on her.
"I thought I felt the wire leave [and] drop about six inches ... I was nervous," Andrews recalled. "So, I called down and I said, 'Excuse me, when you do let me down, could you let me down really gently, because I felt myself slip and I just don't feel too safe up here.'"
The crew called back that they would be sure to bring her down very softly.
That's not what ended up happening.
I plummeted to the stage Julie Andrews
"I plummeted to the stage," Andrews said. "And there was an awful silence for a minute and I did let fly with a few Anglo-Saxon four letter words, I have to admit." The actress smiled at this polished admittance of crassness, much to the delight of the studio audience.
Although Poppins takes flight a few times in the movie, you can likely see the scene Andrews is referring to in her first introduction to the children. Typically when Poppins is flying, the character is completely still (due to rudimentary special effects), but Andrews' body is moving around while flying with the umbrella near the end of this clip: