It called butterfly shrimp because of the
end process, (see image below) which open like wings of a butterfly.
How to butterfly shrimp, you need a cutting board, and a chinese
cleaver (or your favorite knive).
Shelled and clean shrimp, using the cleaver
slit lenght wise until it open up like image below.
Remove vain, if you want the shrimp to look bigger, flatten
the shrimp with the flat side of the cleaver.
This method can be used to thinning thick meat pieces for easier