Just as we may not add on to the mitzvos, we may not detract from them. In the same way that the previous mitzvah meant that we may not add on to any mitzvah that has a particular number involved, this mitzvah means not to reduce such things. For example, the four species taken on Succos include three hadassim (myrtle branches) and two aravos (willow branches) – not four and three, nor two and one. Both increasing and decreasing are equally prohibited.
The Talmud in Rosh Hashana (28b) raises an interesting predicament. Let’s say that blood from an offering that requires one sprinkling gets mixed with blood from an offering that requires four sprinklings. To sprinkle four times violates b’al tosif (adding on) for the former but sprinkling once violates lo sigra (detracting) for the latter. So how should one proceed? In such a case, it’s preferable to sprinkle once then stop. To sprinkle four times would violate b’al tosif actively but to sprinkle once violates lo sigra passively. In such a forced choice, a “sin of omission” is better than a “sin of commission.”
The reason for this mitzvah is the same as for the prohibition against adding on to the Torah: we in no way have any ability to write a better Book than God. For us to re-write the mitzvos that He gave us is a tremendous chutzpah.
This mitzvah applies in all times and places. It is discussed in the Talmud in the tractates of Sanhedrin (88b-89a) and Rosh Hashana (28b). It is codified in the Shulchan Aruch in Orach Chaim 128. This mitzvah is #314 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #160 of the 194 mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.