What is the meaning of "Mecharker"?
Rashi: He dances.
Radak citing Targum Yonasan: He praises [Hashem].
What is the meaning of "b'Chol Oz"?
Radak: With all his strength and intent.
Why did David do so?
Malbim: He nullified his honor due to Hashem's honor in three ways. (a) He danced and ran in front of the Aron. (b) He wore Efod Bad like Kohanim and those who serve Hashem, and not royal clothing, lest he show haughtiness in front of Hashem. In these two ways he was unlike the nation; a greater person must show more submissiveness in front of the Creator, like we say (Brachos 34b) that when a king bows [in Shemoneh Esre], he does not straighten up [until the end of his prayer]. (c) He joined with Yisrael in accompanying the Aron amidst Teru'ah and Shofar blasts (verse 15).
What is "Efod Bad"?
Rashi: It is a linen robe. Yonason translates Efod to be Kardut like he translates Me'ilim (robes) regarding Tamar (13:18).
Radak: It is a garment worn for honor. There were find 85 Kohanim "Nosei Efod Bad" (1:22:18). It was not one of the Bigdei Kehunah, so even a Zar could wear it. In Divrei ha'Yamim I, 15:27 it says Me'il; this is the same as the Efod. However, the Me'il and Efod [of Bigdei Kehunah] in the Torah are different, like the Torah says.
