“subdued.” The Greek word translated “subdued” is epitimaō (#2008 ἐπιτιμάω). Usually epitimaō means to express strong disapproval of someone: rebuke, reprove, censure; or to speak seriously, and thus warn in order to prevent or end an action. It can also mean “punish” (cp. BDAG Lexicon).
In this context, epitimaō has a technical meaning: it is used in Greek religion of gaining control over a spirit, a demon. Robert Guelich (Word Biblical Commentary: Mark) epitimaō can mean “a commanding word uttered by God or by his spokesman, by which evil powers are brought into submission.” Jesus subdued the storm, which was no doubt caused by a demon, by the power of God that he wielded, which he expressed in words. The power came from God and was used by Jesus. Jesus did not gain control over the storm by some “magic words” or formula that he used. “It is not a magical incantation...it is powerful Word of the Son” (Gerhard Kittle, Theological Dictionary, ἐπιτιμάω Vol. 2, p. 626). This storm on the Sea of Galilee is recorded in Matthew, Mark 4:35-41, and Luke 8:22-25, and in every record, epitimaō is used. [For more on epitimaō, and Jesus’ use of the power of God, see the commentary on Mark 1:25.]