This is the method to use when preparing jumbo shrimp (king prawns) for sushi. It will cook them the appropriate amount of time and prevent them from curling.
1. Wash shrimp under running water and cut off heads.
Insert a bamboo skewer or long toothpick along shrimp from head to tail, running along legs of shrimp without touching flesh.
Drop shrimp into a pot of salted, boiling water (use enough salt to make it taste like seawater). Boiling shrimp in salted water keeps protein in shrimp. They will sink to bottom and after 3 to 5 minutes will change color and rise to top. (Do not use a lid, or a strong smell of shrimp will remain.)
To check that they are cooked, remove one shrimp from water and squeeze gently. If inside is firm, it is cooked.
Quickly place shrimp in ice water. This gives them a good color and stops flesh from shrinking and becoming hard. When shrimp are cold, remove from ice water and place in a colander.
To remove skewer, use a screwing motion to avoid breaking flesh.
Remove shell from around body, but not tail.
2. To make butterfly cut, lay shrimp down with tail away from you, then cut from head to tail along belly with knife only going halfway in.
3. Use the knife or your fingers to open out and flatten shrimp carefully, without breaking the flesh.
Remove vein and rinse shrimp with mildly salted water. Lay on paper towels to drain.