Seems to be remnants of an old ceremony that has turned into a pep rally for awakening nationalistic feelings. I don't really know how and why they would have enacted this sort of ceremony at the very beginning.. "Ooh, lets have our border guards throw their thumbs at each other and huff puff their chests for fun". I'm guessing, it was bored border guards coming up with something to do when people/superiors came to see how their loyalty was standing up in front of Indian guards.

Funny nonetheless