When a man wears a silk scarf with a jacket instead of a necktie, tucking the ends into a sleeveless vee necked sweater vest reduces the flamboyance of the combination considerably.
When when one chooses to eschew a necktie this look is probably a less risky choice than an ascot (yes, there are those in the United States who continue to believe that they would be pummeled within an inch of their lives were they seen on the street with an ascot - and we honor their concerns). Like the ascot, it closes the neck, eliminating any glimpse of unsightly undershirt or that hollow at the base of the throat. But the scarf is more common, and unlike the ascot fulfills the practical task of keeping the chest warm at the same time it imparts a bit of silken elegance to the area.
Tuck in your scarf!