Dating gestures mean
It is considered polite to sit seiza-style on formal occasions.
Another more relaxed way of sitting on tatami is cross-legged (agura).
It is commonly done together with the large group of people. It is probably because the Japanese soldiers shouted "Tennouheika Banzai" when they were dying during World War II.
In this context, they meant "Long live the Emperor" or "Salute the Emperor".
It is shouted in happy occasions while raising both arms.
People shout "banzai" to express their happiness, to celebrate a victory, to hope for longevity and so on.
Then, be aware of the many body signs and customs around you.
Westerners may confuse this with a wave and not realize they are being beckoned.
Starting with legs out straight and folding them in like triangles. Women would usually go from the formal to an informal sitting posture by shifting their feet just off to the side (iyokozuwari).
Though most Japanese do not concern themselves with it, it is proper to walk without stepping in the edge of the tatami.
They say a picture paints a thousand words – and the same can certainly be said for gestures.
We all subconsciously give away hints as to our true feelings, through our movements and gestures.