Look first at this very clear and well-illustrated Spanish source on the púa central grafting technique:

"2.1. Injerto púa central
[...]
Se decapita la parte aérea del patrón de 30 a 40 cm. de altura aproximadamente [...]
Seguidamente se procede a partir cuidadosamente el patrón por el centro de 3 a 4 cm. aproximadamente [...]
Inmediatamente se procede a preparar un segmento de vareta que tenga 3 a 4 yemas, para luego hacer dos cortes rápidos y seguros a los laterales en el extremo inferior de tal manera que forme la púa ó cuña [...]
Esta cuña o segmento de vareta se introduce en el patrón partido, haciendo coincidir el acople de las cortezas del patrón con la corteza de la vareta, de tal manera que exista contacto del tejido de patrón y vareta [...]"
http://repiica.iica.int/docs/B4205e/B4205e.pdf (pp. 8-9)

The photos are really helpful.

Another Spanish source, also with photos, though rather less clear:
http://www.fundesyram.info/biblioteca.php?id=3154

Then this English source; it doesn't really describe the technique here, but the photo corresponds exactly to the Spanish source:

"Top-grafting: farmers can graft budwood onto seedlings that have been raised in a nursery. The grafted seedling will be a clone of the mother-tree from which the budwood has been taken, showing the same superior traits. The grafted seedling can be used to replace damaged or diseased cacao trees in the farmers’ cocoa farms."
https://www.swisscontact.org/fileadmin/user_upload/COUNTRIES... (p. 7; p. 4 of file).

And here's an English description, without illustrations:

"[...] Use the sharpknife to make a well tapered V-cut on the scion.
Slit the rootstock the same length as the V-cut on the scion. Remove all leaves from the rootstock. Insert the scion into the rootstock ensuring that all the bud eyes are exposed and that the cambium of both rootstock and scion are in firm contact. With organic thread, secure union making sure that there are no gaps between the scion and the rootstock. [...]"
http://www.worldcocoafoundation.org/wp-content/uploads/files... (p. 64; po. 74 of file).


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Note added at 44 mins (2018-08-01 21:46:30 GMT)
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As you will see if you check, all the sources cited refer to cacao/cocoa grafting.