What does it mean that swallows fly under their chins?

I really haven't heard this sentence, only heard it.

Swallows fly after May, meaning as follows.

1) Swallows fly, chase in May, Erkang picks flowers to send crape myrtle, and ChristianRandPhillips dances. You see, Sister Rong is a villain!

2) Swallows flew, Mei chased after them, Er Kang fell in love with Xia Ziwei, Xiao Jian, who was gentle and sunny, chased after them, the vicious queen stabbed Ziwei, Mammy Rong's chin turned into a turtle, the beautiful golden lock Liu Qing chased after her, and poor Liu Hong didn't. 3) Swallow flies to Brother Wu to chase Er Kang. He likes Xia Ziwei Xia Ziwei kicking shuttlecock and swallows. 4) Swallows fly after May, Fourcand falls in love with Xia Ziwei, the emperor falls in love with ChristianRandPhillips, Xiao Jianfei chases Sunny and the angry queen everywhere.

5) After the swallow flew in May, Erkang fell in love, the emperor waited patiently, and Liu locked a pair, which was pitiful and not suitable (all versions are here, adopt it).