If Y is complex, R is Y with the imaginary part of all elements negated.
If Y is real or non-numeric, R is the same array unchanged, although ⊣ is faster. See Same.
Examples
+3j4 3J¯4 +1j2 2j3 3j4 1J¯2 2J¯3 3J¯4 3j4++3j4 6 3j4×+3j4 25 +A←⍳5 1 2 3 4 5 +⎕EX'A' 1