Calling proc with [() -> void] as a last parameter

[doongjohn]

    import std/sugar
    
    # if [() -> void] is a last parameter of a proc you can do this(?)
    proc lambdaTest(a: int, lambdaProc: () -> void) =
      lambdaProc()
    
    lambdaTest(1):
      echo "[() -> void] as the last parameter."
    
    
    Run

Just curious

How does ":" syntax work?