diff --git a/src/runProc.cpp b/src/runProc.cpp
index d7af5e8..a7783d8 100644
--- a/src/runProc.cpp
+++ b/src/runProc.cpp
@@ -49,6 +49,9 @@ void T_runProc::runProc_slotProcess(void)
 {
 #ifndef THIS_IS_CA_MASTER
     bool const coinAttached = epi_getDI_CoinAttach();
+    // exchange: (atomically) replaces the underlying value of m_coinAttached
+    // with the value of coinAttached, returns the value of m_coinAttached
+    // before the call
     if (m_coinAttached.exchange(coinAttached) == false) {
         if (coinAttached) {
             // old value was false, and new value is true