While flying in a gully, with me on a ridge, higher than P2v+, I contacted the ground, not at speed but bird got wrapped up in some long grass. The height was simular to the take off height, but some 400m away from me. I tried to kill engines once contacted, but they wouldn't shut down. Finally when I got to machine (gasping for breath) 2 blades where spinning happily, 2 were jamed up with grass, but still trying to turn. I had to clear it from the debris and then it turned off with my controller. Any idea as to why? 2 stalled motors were fairly hot, so I restarted to allow air to flow over motors. I have flown since with no problems. What is another way of killing engines or why didn't they shut down? Both sticks in the centre bottom left and right postion, I used sw1 incase it was trying to RTH at one point, made no differance.