(5:4)"And the Jews did so, and they sent them [those who were tameh, had tzora'as or were zavim] out of the camp as Hashem had told Moshe". There are many occasions in which the Torah reports that obligations were performed in accordance with Hashem's instructions, and all are appropriately noted as "ka'asher tzivah" - as Hashem commanded. However in an observation that only Rav Meir Simcha of Dvinsk could make, he notes that the pasuk quoted here is the only time in Torah where Hashem's instructions were carried out "ka'asher dibeir" - as Hashem spoke. Throughout all of Noah's preparations for the Mabul, he is described as following the command of Hashem, "And Noah [brought all the animals into the Teivah] - ka'asher tzivah - as Hashem commanded him" (6:22). "And Avraham performed the bris on his son Yitzchak,who was eight days old - ka'asher tzivah - as Hashem commanded him" (21:4). "And they went and they [brought the korban pesach] as Moshe and Aharon were commanded (Ex12:28)". The record of the gifts that were brought for the building of the mishkan is punctuated with the refrain, ka'asher tzivah Hashem - as Hashem commanded. Why then is the reference to the mitzvah of sending some individuals who are tameh out of the camp, said to be observed ka'asher diber - as Hashem spoke.
Rav Meir Simcha in his commentary to the Torah, Meshech Chochma, answers that the word "tzivah" does not simply mean command. Rather, ka'asher tzivah implies encouragement, prodding, perhaps even demanding. Ka'asher tzivah indicates acts of obeisance alone, which are void of any anticipation of great satisfaction and certainly empty of any self serving consequences. Indeed Rashi at the beginning of Parshas Tzav reminds us that the very usage of "tzav" indicates that the loss of money associated with the korban olah required greater encouragement than other mitzvos. However, remarks the Meshech Chochma, sending the tameh out of the camp, was welcomed by the tameh as it would achieve kaparah, forgiveness and a reinstatement of his position in the eyes of Hashem. That did not require any additional encouragement and was carried out, "ka'asher dibeir," as something unceremoniously taught to Moshe, with no push or fanfare.
It would seem to me that there is another insight lurking in the Meshech Chochma's observation. Mitzvos that are observed "ka'asher tzivah", are by virtue of the added encouragement, informed with the vigor, passion and pride of one who sees oneself as doing the Creator's bidding. One can only imagine that all the misgivings that Noach had about the oncoming destruction notwithstanding, he must have been exhilarated to be chosen to ensure the survival of all that Hashem had wrought. Surely the paternal pain felt at the time of Yitzchak's bris was overwhelmed by the joy that filled Avraham, as he witnessed the continuation of all that he sought to bring into this world. There is no question that the bris was performed ka'asher tzivah. Can one doubt that the Jews leaving Egypt were joyous beyond description as they brought the Korban Pesach signaling their nascent redemption?
However there is one mitzvah that we understood on our own to carry out in a begrudging manner - "ka'asher diber", because He said so, as an act of obeisance. Asking someone to leave the camp, even if it is because he is tameh and even if he welcomes the opportunity for kaparah, is one of the hard realities of communal life that Hashem has on occasion demanded of us. The Jews understood that the manner in which this mitzvah is exercised must communicate the hesitation always inherent in distancing Jew from Jew. When a destructive attitude or a negative influence has to be separated away, it must be done with the visible pain of someone doing one's duty and devoid of the satisfaction that should otherwise highlight all service to Hashem. In so doing one remains true to the love of every Jew that must guide all interactions with others.