Pulse,
Press and hold the button and you have sole control of the aircraft, normally accompanied by a clear statement of "I have control". The other stick stops working, and the other pilot gets an alerting light on his glare shield. Release the button and both sticks become useable again.
The 40 second function is to disable the other stick completely, for example if it gets stuck or is being leaned on by an incapacitated pilot. After 40 seconds you can release your button and the other stick remains inop. So much quicker and easier than trying to separate cross-connected columns and yokes in similar circumstances...