The clocks in this situation are all quartz clocks with hands. The time base is provided by quartz crystal vibration. If there is electricity, the clock will go without electricity. The slowness here is caused by mechanical drag.
Pointer with the quartz clock is stepping motor, every time a fixed Angle, similar to walk down on floor tile of obsessive-compulsive disorder, it is impossible to get out of the half step (benefits in this motor is to control the Angle, or turns absolutely accurate, precise brake in time, there is no control ran out of the 1.001 times, especially suitable for clock). Even if the low power motor runs out of power, it still has to go one step further.
The slow speed usually occurs when the second hand walks to the left side of the clock face, because the gravity of the pointer makes the motor need to output greater force to turn, the motor will not be strong enough, and will hold for a few seconds, the convulsion may be slow down to continue to walk. At this point the heavy horizontal time can go for a period of time.
When there is almost no electricity, even the strength of the motor is too weak to overcome the resistance inside the movement, so only the second hand can be seen swinging.