It must be clear that if the arrestee ends up (or is found) on his back, then the standing armlock is used only as a quick transitional position given his ability to use his free arm and legs to attack you (or even to spit on you) as he is facing you.
You can also spin him around using his arm as a tether to better position yourself to surrounding threats.
PRO TIP: Always walk around the top of his head; never step across his body as you can be attacked and this move will confound your joint-locking control during the rollover (i.e., his arm will be twisted in the wrong direction to take his back).
The use of the bent wristlock (with his fingers always pointing at his head) is commonly used to move from the supine standing armlock position to the prone standing armlock position.
Seize his arm firmly between your legs and use a bent wristlock in combination with a straight armbar, using either of your inner thighs/knees as fulcrum.
Maintain the bent wristlock (with his fingers always pointing at his head) using a downward posting action of his straight arm shoulder into the ground (aided by the bent wristlock) while you kneel 45 degrees down his back to help keep him in the prone position.
Other methods of assisting the rollover include the use of your knuckles (on the triceps tendon) or knee to the elbow when executing a bent-wrist lock rollover, both creating an arm bar to give him incentive to complete the roll.
The triple twistlock (handshake) offers the very easiest way to execute a rollover.
Once he is rolled over, assume the prone standing armlock position, or simply go to the kneeling prone handcuffing position using a bent wristlock/knee capture to control his arm.