To poise something means to position it or to arrange it in such a way that it will remain where it was positioned or remain as it was arranged; there is often the sense that it could easily be knocked from that position or out of that arrangement--the stasis is fragile or precarious.
The 'something' which is poised can be one's own person, one's body, or one's mind, attitude, or mental state, if the word is being used figuratively.
The slapstick comic poised himself on the ladder, holding a flower-pot
in his extended arms, waiting for his fellow stooge to walk through
The workers poised the A/C unit precariously on the window sill.
He poised the teaspoon on the tip of his nose.
He poised the broomstick on the tip of his finger and tried to walk across the room.
She poised the mannequin's arm in a languid gesture.
In your second example, the verb is being used figuratively to refer not to an actual physical place or physical arrangement, but a mental attitude or state.
He stood poised between steely self-control and utter abandon.
There, even though we find the verb stood and the preposition between, we are not talking about physically standing on two feet with something on either side, but about a state of mind, and two equally powerful attitudes or motives tugging the person's mind in different "directions", so that it is held "motionless", in equipoise.