拉勾 (lāgōu, 'pinky promise') is exactly the gesture you know; children link
小拇指 and chant
拉勾上吊一百年不许变 ('pinkies linked, swearing, no changing for a hundred years!').
小拇指 is also slang for 'the smallest/weakest one': 'I'm just a
小拇指 in this team' = 'I'm the rookie.'
