This is a (EM style) magic wall. It gets activated for a limited
-time by certain elements that fall on it. Objects falling though
+time by rocks or gems that fall on it. Objects falling though
it will be changed to other objects. After it has stopped running,
it cannot be activated again.
+
+The duration is expressed in seconds. A duration of zero will let the wall
+run forever. All regular magic walls run together; however, BD style magic
+walls have a separate counter.