http://www.mangago.zone/read-manga/noda_ke_no_oshie_ni_yori/uu/noda_ke_no_oshie_ni_yori-ch...

he's strengthening his leg muscles!!! lmao but it's very funny he does it at the dinner table...

that’s called ‘kuuki isu’ or ‘air chair’
Kids in Japan do it for fun as a joke, but dad seems to be doing it seriously.