diff --git a/doc/usage.md b/doc/usage.md
index cb0d1929..dbfc42d6 100644
--- a/doc/usage.md
+++ b/doc/usage.md
@@ -54,7 +54,9 @@ Handle to cothread.
 
 Handle must be of type `void*`.
 
-A value of `null` (0) indicates an uninitialized or invalid handle, whereas a non-zero value indicates a valid handle.
+A value of null (0) indicates an uninitialized or invalid handle, whereas a
+non-zero value indicates a valid handle. A valid handle is backed by execution
+state to which the execution can be co_switch()ed to.
 
 ## co_active
 ```c
@@ -62,7 +64,12 @@ cothread_t co_active();
 ```
 Return handle to current cothread.
 
-Always returns a valid handle, even when called from the main program thread.
+Note that the handle is valid even if the function is called from a non-cothread
+context. To achieve this, we save the execution state in an internal buffer,
+instead of using the user-provided memory. Since this handle is valid, it can
+be used to co_switch to this context from another cothread. In multi-threaded
+applications, make sure to not switch non-cothread context across CPU cores,
+to prevent any possible conflicts with the OS scheduler.
 
 ## co_derive
 ```c