Which buddhist traditions use silent visual applause, instead of clapping, to show appreciation and what is the origin of this practice?
This video says that it's widespread among French and German university students, ecologists, especially left-wing, for example at political assemblies and unions. And it says that an internet search shows that it's part of the "language of signs" but that no-one knows which country it began in.

The page Monks and nuns from the Thich Nhat Hanh collective, in Plum Village, France, pay our school a special visit suggest that Thich Nhat Hahn are:


From France
Teaching 'silent applause'
Teaching 'ecologist' songs such as "I love nature"


So perhaps they adopted the gesture from that milieu (or vice versa): i.e. from European students and young ecologists.