Never write your chances off, no matter how slim they appear.
The 21-year-old barely made the cut for the second leg after a mistake late on in his first descent of the Männlichen piste, finishing 29th and earning himself the second starting position for the afternoon run.
But a blistering run in tricky conditions – no-one came within more than nine tenths of him in the afternoon – put him provisional first.
And he watched with increasing incredulity as skier after skier, the favourites, crashed out or lost time in warming conditions above freezing that made the snow in the middle and final sectors extremely soft.
His total time of 1 minute 41.48 seconds held firm, giving the Norwegian only his second career World Cup win.
"Coming into the day's race the main goal was to keep up my stability, not race too much and just do technically [good] skiing," a stunned Braathen told FIS afterwards.
"With my mistake in the first run I was eager to redeem myself in the second. I didn't even know it was possible to get a win from [29th] in the first run. Returning from the injury last season, words can't describe how grateful I am for this right now.
"Wengen is one of my favourite places on Earth to ski, it's so unique. The support was * amazing."
Switzerland's Daniel Yule** took second place, while Italian Vancouver 2010 Olympic champion Giuliano Razzoli finished third for his first World Cup podium since he finished second on the same hill in 2016.
The trick to it all, perhaps, was that the course for the second run was set by the Norwegians, in favour of their skiers Braathen, slalom red bib holder Sebastian Foss-Solevaag, and first-run leader Henrik Kristoffersen.
Coupled with the warm temperatures, which left the top of the course – covered in shade – bumpy and rutty while the last third of the run was bathed in sunshine and softening by the minute, an early start in the second run proved crucial.
Braathen was magnificent, and only two other men – Yule and Kristoffersen – could match his pace through the first timing split.
Yule, racing in front of home fans, did begin to lose time down the remainder of the course and found himself 0.22 seconds outside at the finish. He could be heard telling Braathen: "That little section is really tough. Amazing skiing."
When Foss-Solevaag, Braathen's mentor, crossed the line outside Braathen's time to confirm the latter's spot on the podium, the youngster reacted in shock and began crying as his achievement began to dawn on him.
Austria's Manuel Feller then could only place fifth, ensuring Braathen would be in the top two.
Kristoffersen, who held an advantage of 2.04 seconds from the first run, would be last down the slope. After matching Braathen through the first sector, he began losing time, but it still appeared that he would comfortably take victory.
Until he didn't – Kristoffersen had a momentary lapse of concentration with the finish line in sight, straddling a gate in the final sector while still more than a second up, ending his run and sending Braathen to his knees in disbelief.