I don't ride with Shimano's SPD MTB pedals, I use the Time ATAC ones so I'm not familiar with exactly how the Shimano ones work. I'm pretty sure though (although could be corrected if wrong) that even if they are set for a light release, they won't release if you are pulling them straight up but will only release if you turn your heel out. If so, that's part of the training of riding with clipless.

J.