diff --git a/src/concurrency/send-sync.md b/src/concurrency/send-sync.md
index 4b6fa237..b179e6b2 100644
--- a/src/concurrency/send-sync.md
+++ b/src/concurrency/send-sync.md
@@ -7,5 +7,16 @@ How does Rust know to forbid shared access across thread? The answer is in two t
 * [`Sync`][2]: a type `T` is `Sync` if it is safe to move a `&T` across a thread
   boundary.
 
+Both traits are not to be implemented. They are implemented automatically when 
+the compiler determines it’s appropriate.
+
 [1]: https://doc.rust-lang.org/std/marker/trait.Send.html
 [2]: https://doc.rust-lang.org/std/marker/trait.Sync.html
+
+<details>
+
+* One can think of these traits as markers that the type has certain
+  thread-safety properties.
+* They can be used in the generic constraints as normal traits.
+  
+</details>