diff --git a/BUILD.md b/BUILD.md
index 2f08f36..7ed548f 100644
--- a/BUILD.md
+++ b/BUILD.md
@@ -139,9 +139,17 @@ statements when `--enable-debug` is not used.
 ```
 --with-assert=trap
 ```
-Recommended when using `--enable-assert` for debugging. This replaces the
-default mechanism of assertion with traps rather than aborts; allowing
-developers to explore an unterminated program.
+Recommended when using `--enable-assert` for functional-testing and debugging.
+This replaces the default mechanism of assertion with traps rather than aborts;
+allowing developers to explore an unterminated program.
+
+```
+--with-assert=opt
+```
+Optimistic assertions are branchless and generate less code than other modes.
+Assertion failure is only detected at the end of each core event loop iteration.
+These assertions are intended to minimally reveal the existence of a failure and
+are much harder to debug.
 
 #### Manually enable optimization