The table below presents an aligned extract of verses or verselets in Dt04-06b Both verses/verselets discuss the requirement to observe the commandments. The alignment justifies the Rashi comment that: The primary effect of the Sinaitic revelation is the commandment to observe. But observance itself requires safeguarding the commandments you observe. One safeguards the commandments by professionally standardizing them; these standards are called fences in Rabbinic law. For example the Bible commanded to recite the Shma in the evening. Rabbinic law standardizes this practice so that it must be recited before midnight (otherwise people would fall asleep and miss the recital of the Shma.).

Verse	Text of Verse	Rashi comment
Dt04-05	Behold, I have taught you statutes and judgments, as the Lord my God commanded me, that you should do them in the land where you go to take possession of it.	The primary effect of the Sinaitic revelation is the commandment to observe. But observance itself requires safeguarding the commandments you observe. One safeguards the commandments by professionally standardizing them; these standards are called fences in Rabbinic law. For example the Bible commanded to recite the Shma in the evening. Rabbinic law standardizes this practice so that it must be recited before midnight (otherwise people would fall asleep and miss the recital of the Shma.).
Dt04-06	You should watch/safeguard and do them; for this is your wisdom and your understanding in the sight of the nations, who, when they shall hear all these statutes, shall say, Surely this great nation is a wise and understanding

Advanced Rashi: Another approach to this Rashi is the understanding of the meaning of the Hebrew root, Shin Mem Resh. We can show that in the Bible Shin Mem Resh refers to professional obligations and/or to professional standards. If you think about it a little bit, professional standards simply means fences preventing violation.