Ps 141 5 Let a righteous man strike me-it is a kindness;
let him rebuke me-it is oil on my head.
My head will not refuse it."

The Psalmist never prays for God to wound him and break him..never. The word strike in the Psalms is always used for rendering the enemy powerless. this is a play on that usage. He is using this word in the context of war. In other words we can compare this to a man who is fighting and then the enemy comes up behind him to kill him. But the righteous man steps in between the enemy and the Psalmist and strikes the enemy with a fatal blow. The Psalmist is saying that we need our bothers to protect us when we there is a potential for us to be harmed. Strike is always used as a fatal blow that is near to Gods elect