"it shall you observe (tishmor) to do": R. Eliezer b. Yaakov says: (The intent is) to assign a negative commandment to all that is mentioned in this section.
"you shall not add to it and you shall not detract from it": From here they ruled: If (the blood of offerings) which are subject to one application (of blood on the altar), became intermixed with (the blood of others) which are subject to one application, one application should be made (sufficing for both). If (the blood of) offerings which are subject to four applications became intermixed with (the blood of others) which are subject to four applications, four applications should be made (sufficing for both). If (the blood of offerings) became intermixed with (the blood of offerings) which are subject to one application — R. Eliezer said: Four applications should be made; R. Yehoshua said: One application should be made. R. Eliezer: But (according to you) he transgresses "you shall not detract!" R. Yehoshua: But (according to you) he transgresses "You shall not add!" R. Eliezer: "You shall not add" applies only when it is alone (and not intermixed with the blood of another which is subject to four). R. Yehoshua: "You shall not detract" applies only when it is alone (and not intermixed with the blood of another which is subject to one). And R. Yehoshua said further: By applying (four), you transgress "you shall not add" — by performing an action (of addition), whereas by not applying four, you have, (indeed) transgressed "you shall not detract," but you have not performed an action.