diff --git a/share/man/man7/tuning.7 b/share/man/man7/tuning.7
index 44c427c4559..a3709ce82b8 100644
--- a/share/man/man7/tuning.7
+++ b/share/man/man7/tuning.7
@@ -222,6 +222,21 @@ and
 .Va vm.stats.vm.v_wire_count
 sysctls, respectively).
 .Pp
+Due to the architecture of the
+.Fx
+virtual memory subsystem, the use of copy on write (CoW) anonymous
+memory, e.g. on
+.Xr fork 2 ,
+causes swap reservation for all three regions (VM objects),
+as in the original pre-fork mapping, and its copies in
+the parent and child, instead of only two.
+Eventually the subsystem tries to optimize the internal layout
+of the tracking for CoW and often removes (collapses) no longer
+needed backing objects, re-assigning its pages and swap
+reservations to the copies.
+Coolapsing frees the swap reserve, but it is not guaranteed
+to happen.
+.Pp
 The
 .Va kern.ipc.maxpipekva
 loader tunable is used to set a hard limit on the