in my understanding and playing experience, going dead can mean two different things, basically both related to a loss in elasticity, or better said resiliency.
1. a string that after a certain amount of time just gets like a board so you have to hit faster and faster in order to get the ball over the net. stringbed deflection is a minimum but recoil time is also delayed as in case no.2.
2. a string that turns into a slingshot, that is apparently still has elasticity left but responds with a certain time delay to the impact and makes the ball basically uncontrollable. in this case there is still enough stretch in the stringbed, but it has a very delayed response. it also seems that dwell time would be higher, but this is not beneficial in such a case.