diff --git a/speedtest-hd.py b/speedtest-hd.py
index 96090fc..40dbc60 100755
--- a/speedtest-hd.py
+++ b/speedtest-hd.py
@@ -715,6 +715,10 @@ def main(argv: Optional[Sequence[str]] = None) -> int:
 
     confirm(cfg)
 
+    # Dictionary dispatch: map each mode name to its handler function (the values
+    # are the functions themselves -- no parentheses), look up the one for `mode`,
+    # then call it with (cfg). Equivalent to an if/elif chain over `mode`. `mode`
+    # is always one of these three keys by now, so the lookup can't KeyError.
     {"cdm": cdm_profile, "slog": slog_profile, "dd": dd_profile}[mode](cfg)
     return 0