slowing down at a stoplight in overdrive goin 15 mph friends foot from the back seat kicks the shifter into reverse slows way down and car dies starts right back up and goes through the gears fine havnt really got on it to see if goes good wot but it drives fine undernormal accleration but what could have broke or got messed up?

 

That will usually kill the tranny, but you might have gotten lucky. As long as it shifts fine you should be ok.

 

As car comes to a stop, I pull idiot friend out of backseat and proceed to pummel his brain into reverse.

You were probably goin slow enough not to kill anything but its still not good for it.
Tell him to keep his feet on the floor or he can walk to the next place.

 

I did the same thing and it broke my tranny mount. you would know if that broke though. It felt like the damn tranny was gonna come through the floor. Extremely loud thumping.