to crack one's knuckles or cross the fingers, stretching them out (see Molina)
IPAspelling:
kɑkɑpɑːniɑ
Alonso de Molina:
cacapania. nino. (pret. oninocacapani.) restallar o hazer cruxir los dedos estirandolos. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 10v. col. 1. Thanks to Joe Campbell for providing the transcription.