Philip Yancey - "PRAYER Does it Make Any Difference?" notes:'Be still and know that I am God" The Latin imperative for "be still" is vacate As Simon Tugwell explains, "God invites us to take a holiday [vacation], to stop being God for a while, and let him be God."... the first step to prayer is to acknowledge or "remember" God -- to restore the truth of the universe. "That man may know he dwells not on his own," said Milton. (pages 26 and 27)
Notes found on line at website noted below :
First, the injunction to "Be still" must be understood in the milieu it was uttered. The Psalmist addressed a cosmos in crisis. The crisis imperiled the creation (vv. 1-3); threatened the city (vv. 4-7); and besieged the country (vv. 8-11). In the crisis with their world falling apart, the people were afraid (v. 2).
Second, the verb "Be still" (Hebrew, rapah) is used 46 times in the Old Testament with meanings everywhere from describing laziness to ordering relaxation. Though the majority of versions translate the injunction "Be still", other meanings are "Cease striving " (NASB), "Be quiet" (NCV), "Desist" (Young's), or "Calm down" (CEV). In no biblical usage or context does the Hebrew verb enjoin God's people to meditate or contemplate. Rather, believers are to rest and trust in God.
Third, verse 10 contains two co-ordinate imperatives, with the emphasis being on the second command, to "know that I am God", not the first, to "Be still". With the first imperative functioning as an adverb, the verse might read, "Calmly (or quietly) know that I am God . . ."  Thus by their focusing upon the initial command, to "Be still," comtemplative spiritualists ignore the greater command, and that is, to "know that I am God."
The command "know," primarliy means, "to know by observing and reflecting (thinking) . . ." As such, believers are encouraged to find comfort of soul by reflecting upon the saving works that God has both performed and promised. The meditation the psalm envisions is therefore objective, not subjective. "Be still" does not call persons to induce within their consciousness a wordless void or incubator in which state a mystical experience or word can be hatched. The cognitive command to "know" cancels that notion. In the light of God's mighty works and providence, the psalm exhorts believers to reverence Him. As the prophet Habakkuk wrote, " . . . the Lord is in His holy temple. Let all the earth be silent before Him" (2:20).
Be still, and know that I am God...
(Latin: from vacare, "to empty")
A partial evacuation.
1. A reference to emptying; evacuative; purgative; cathartic.
2. Medicine which tends to empty an organ or passage.
3. Evacuating; promoting thorough evacuation; an evacuant medicine or agent; especially, from the bowels; being cathartic; purgative.
evacuate, evacuating, evacuated
1. To leave empty; to vacate.