Bring a pot of water, with a dash of salt, to a boil.
While waiting for the water to boil, insert a bamboo skewer under the belly shell of the shrimp starting at the head and ending at the tail. The purpose of this is to straighten out the shrimp and keep it straight while cooking.
Once our pot of water is boiling, add the skewered shrimp to the water cook until the shrimp turns pink and is firm under its shell. This usually takes around 3 minutes.
While the shrimp is cooking, prepare a bowl of ice water to add the shrimp to when they are done cooking.
Once the shrimp is cooked, add to your ice bowl and keep there until cool (usually a minute or two).
Once cool, take the shrimp out of ice water, remove the bamboo skewers and the shells on the body, leaving only the tail.
Using a sharp knife, butterfly the shrimp along the belly. You can do this by making a cut in the belly of the shrimp along its entire length (from head to tail), making sure to only cut to the vein of the shrimp. Once your cut is made, spread the shrimp open and clean out the vein.
Pat dry before using in your nigiri.