They under-doped certain areas to create depletion mode transistors that any reverse engineer examining the silicon would expect are regular enhancement mode ones. They strategically placed a handful of these "disguised" transistors, which once incorrectly copied would switch in the opposite manner and wreak havoc on circuit operation.
According to one of the designers[1]:
I heard from NEC later the copy maker delayed the announcement of Z80 compatible product for about six months.
As the above images, and the presence of many Soviet clones shows, the obfuscation was not entirely impossible to detect.