diff -uNrp rev1/include/SDL_haptic.h rev2/include/SDL_haptic.h
--- rev1/include/SDL_haptic.h	2014-08-23 20:24:23.217689665 +0200
+++ rev2/include/SDL_haptic.h	2014-08-23 21:45:09.573875988 +0200
@@ -553,7 +553,7 @@ typedef struct SDL_HapticPeriodic
 
     /* Periodic */
     Uint16 period;      /**< Period of the wave. */
-    Sint16 magnitude;   /**< Peak value. */
+    Sint16 magnitude;   /**< Peak value; if negative, equivalent to 180 degrees extra phase shift. */
     Sint16 offset;      /**< Mean value of the wave. */
     Uint16 phase;       /**< Horizontal shift given by hundredth of a degree. */
 
diff -uNrp rev1/src/haptic/darwin/SDL_syshaptic.c rev2/src/haptic/darwin/SDL_syshaptic.c
--- rev1/src/haptic/darwin/SDL_syshaptic.c	2014-08-17 00:20:14.000000000 +0200
+++ rev2/src/haptic/darwin/SDL_syshaptic.c	2014-08-23 21:01:52.426068731 +0200
@@ -23,6 +23,7 @@
 #ifdef SDL_HAPTIC_IOKIT
 
 #include "SDL_assert.h"
+#include "SDL_stdinc.h"
 #include "SDL_haptic.h"
 #include "../SDL_syshaptic.h"
 #include "SDL_joystick.h"
@@ -872,9 +873,10 @@ SDL_SYS_ToFFEFFECT(SDL_Haptic * haptic,
         SDL_memset(periodic, 0, sizeof(FFPERIODIC));
 
         /* Specifics */
-        periodic->dwMagnitude = CONVERT(hap_periodic->magnitude);
+        periodic->dwMagnitude = CONVERT(SDL_abs(hap_periodic->magnitude));
         periodic->lOffset = CONVERT(hap_periodic->offset);
-        periodic->dwPhase = hap_periodic->phase;
+        periodic->dwPhase = 
+                (hap_periodic->phase + (hap_periodic->magnitude < 0 ? 18000 : 0)) % 36000;
         periodic->dwPeriod = hap_periodic->period * 1000;
         dest->cbTypeSpecificParams = sizeof(FFPERIODIC);
         dest->lpvTypeSpecificParams = periodic;
diff -uNrp rev1/src/haptic/windows/SDL_dinputhaptic.c rev2/src/haptic/windows/SDL_dinputhaptic.c
--- rev1/src/haptic/windows/SDL_dinputhaptic.c	2014-08-17 00:20:14.000000000 +0200
+++ rev2/src/haptic/windows/SDL_dinputhaptic.c	2014-08-23 21:01:46.792030148 +0200
@@ -21,6 +21,7 @@
 #include "../../SDL_internal.h"
 
 #include "SDL_error.h"
+#include "SDL_stdinc.h"
 #include "SDL_haptic.h"
 #include "SDL_timer.h"
 #include "SDL_windowshaptic_c.h"
@@ -714,9 +715,10 @@ SDL_SYS_ToDIEFFECT(SDL_Haptic * haptic,
         SDL_memset(periodic, 0, sizeof(DIPERIODIC));
 
         /* Specifics */
-        periodic->dwMagnitude = CONVERT(hap_periodic->magnitude);
+        periodic->dwMagnitude = CONVERT(SDL_abs(hap_periodic->magnitude));
         periodic->lOffset = CONVERT(hap_periodic->offset);
-        periodic->dwPhase = hap_periodic->phase;
+        periodic->dwPhase = 
+                (hap_periodic->phase + (hap_periodic->magnitude < 0 ? 18000 : 0)) % 36000;
         periodic->dwPeriod = hap_periodic->period * 1000;
         dest->cbTypeSpecificParams = sizeof(DIPERIODIC);
         dest->lpvTypeSpecificParams = periodic;
diff -uNrp rev1/test/testhaptic.c rev2/test/testhaptic.c
--- rev1/test/testhaptic.c	2014-08-17 00:20:14.000000000 +0200
+++ rev2/test/testhaptic.c	2014-08-23 22:33:59.242997030 +0200
@@ -122,7 +122,8 @@ main(int argc, char **argv)
         SDL_Log("   effect %d: Sine Wave\n", nefx);
         efx[nefx].type = SDL_HAPTIC_SINE;
         efx[nefx].periodic.period = 1000;
-        efx[nefx].periodic.magnitude = 0x4000;
+        efx[nefx].periodic.magnitude = -0x2000;    /* Negative magnitude and ...                      */
+        efx[nefx].periodic.phase = 18000;          /* ... 180 degrees phase shift => cancel eachother */
         efx[nefx].periodic.length = 5000;
         efx[nefx].periodic.attack_length = 1000;
         efx[nefx].periodic.fade_length = 1000;