There are several types of Shimano SPD cleats. Among them are "single release" and "multiple release". The single release only comes out by twisting the heel. The multiple release will come out in any direction with overload. I use single release cleats and the only time I come out unintentionally is when I twist my foot inadvertently. Check the second page of the following Shimano instruction sheet for details on the two types:

http://techdocs.shimano.com/media/te...9830617303.pdf