Hold crabs firmly. Slide a sharp, strong knife under the top of the shell at the back, lever off the shell. Remove and discard the whitish gills. Rinse well under cold water. Crack nippers slightly and cut crabs in half.
In a large bowl, combine crabs with remaining ingredients, cover and refrigerate for at least 20 minutes.
Cook undrained crabs on heated oiled BBQ, uncovered, until changed in colour, turning occasionally.