diff --git a/RP_L4/RPL4ENVIRON.cpp b/RP_L4/RPL4ENVIRON.cpp
index 0f1764c..4f122a9 100644
--- a/RP_L4/RPL4ENVIRON.cpp
+++ b/RP_L4/RPL4ENVIRON.cpp
@@ -199,6 +199,29 @@ namespace
 "# default is 60; the arcade pods shipped at 25.\n"
 "TARGETFPS=60\n"
 "\n"
+"# The physics step, in steps per second - the option under test.\n"
+"#\n"
+"# 0 (the default) is the game as it has always run: the simulation\n"
+"# advances by however long each frame took, so the frame rate IS part\n"
+"# of the physics - a pod at 30 fps falls, corners and collides\n"
+"# differently from one at 144. Any other value here decouples them:\n"
+"# the simulation advances in fixed steps of exactly this rate whatever\n"
+"# the display does, and the SAME race plays out on every machine -\n"
+"# measured bit-identical at 30, 60 and 144 fps.\n"
+"#\n"
+"# The three rates worth testing, all exact on the engine's millisecond\n"
+"# clock:\n"
+"#   25    the arcade pods' rate - the step the original handling was\n"
+"#         tuned against, coarsest contact response\n"
+"#   50    the middle road\n"
+"#   100   the smoothest contact and terrain response\n"
+"# (Rates that do not divide 1000 evenly - 60, say - quietly run at the\n"
+"# nearest millisecond step instead; the log says so if you try one.)\n"
+"#\n"
+"# What to feel for against 0: hover bounce, wall hits, how the pod\n"
+"# takes the crest of a hill. Report the rate with the verdict.\n"
+"#RP412PHYSICSHZ=50\n"
+"\n"
 "# How long one background pass may spend drawing cockpit gauges, in\n"
 "# milliseconds. The gauges and the MFD/map displays are redrawn in the\n"
 "# time left over after the 3D view; on a big, busy map there is none\n"