diff --git a/RP_L4/RPL4ENVIRON.cpp b/RP_L4/RPL4ENVIRON.cpp
index ad65440..d449b8a 100644
--- a/RP_L4/RPL4ENVIRON.cpp
+++ b/RP_L4/RPL4ENVIRON.cpp
@@ -238,6 +238,23 @@ namespace
 "# takes the crest of a hill. Report the rate with the verdict.\n"
 "RP412PHYSICSHZ=50\n"
 "\n"
+"# 0 = draw the simulation's exact stepped positions, as builds before\n"
+"# this one did. On by default, and it is what lets the frame rate be\n"
+"# anything you like.\n"
+"#\n"
+"# With a fixed physics step the drawn position only changes at the step\n"
+"# rate, so any other frame rate holds each position for a whole number\n"
+"# of frames and the motion visibly steps - worse the FASTER the machine,\n"
+"# since 240 fps holds each one for nearly five frames. Drawing therefore\n"
+"# blends across the step it is inside. The simulation is not touched: the\n"
+"# same script drives the same race bit for bit either way, which is what\n"
+"# RP412PHYSTRACE below is for.\n"
+"#\n"
+"# The cost is that the picture trails the simulation by up to one step\n"
+"# (20 ms at 50 Hz). Turn it off to see the stepping it removes, or if you\n"
+"# would rather have those 20 ms than smooth motion.\n"
+"#RP412INTERP=0\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"