How Stringent are Rabbinical Laws?

The Talmud appears to send mixed messages as to the stringency of rabbinical laws. On the one hand, the general rule seems to be that rabbinical law is more leniently applied than Torah law, as seen in the context of safek (indeterminate circumstances) and other areas. On the other hand, the Talmud states in several places that the Rabbis “strengthened their words more than words of Torah”, and in Berakhot 4b we are told a particularly harsh warning, that one who violates rabbinical law is deserving of the death penalty. As the Talmud explains, this severity emanates from the greater likelihood that the law under discussion will be treated lightly, which is also the reason given as to why the Rabbis at times strengthened their laws more than those of the Torah. However, it remains unclear when this attitude is applied. The Muncaczer Rebbe (Resp. Minchat Elazar, III, 38) suggests that the guiding issue is whether or not the rabbinical law in discussion has any source or asmakhta in biblical law. If it does, then no strengthening is needed or appropriate. If, however, the law is a wholly rabbinic innovation, extra support becomes necessary. (The actual application of that distinction is still complex and requires further elaboration, and that is the topic of the rest of the responsum.)