Before the advent of the thyristor which changed the way speeds on electric trucks were controlled, the standard technique was to arrange for a combination of resistors and contactors to control speed. A resistor simply "resists" the passage of electricity turning it into heat and a contactor is simply a big switch. A typical circuit is shown here.
Imagine two wires connecting a battery to a motor. In series with the wires are one, two or three resistors depending upon how many "gears" are required. Each resistor can be shorted out by a contactor which is suitably connected to do this.
Lets take a common battery voltage - 48 volts fitted to many trucks - and see what happens in actual operation.
The operator puts his/her foot on the accelerator and yet another contactor closes to complete the circuit between the battery and truck drive motor. The electricity at this point is passing through the resistors which reduce the voltage and turn it into heat instead.
As the operator presses the accelerator down, a mechanical system causes the contactors to close effectively removing the resistors from the circuit one at a time. If we assume that each resistor cause (say) 12 volts to be "lost" there would only be 12 Volts left to go to the motor if all three resistors were in the circuit. This voltage would probably just about cause the truck to move albeit very slowly.
As resistor number one is removed when the contactor shorts it out, so the voltage applied to the motor goes up - say to 24 volts. Short out another and the voltage increases to 36 and now finally close the remaining contactor and the battery is connected directly to the drive motor which then receives the full 48 volts allowing the truck to travel at its maximum speed.
This system is relatively cheap and reliable but suffers from two problems:
1) A lot of heat is generated so special steps have to be taken to ensure that the controller doesn't overheat. I've seen them set on fire in the past!
2) More important is the fact that the driver does not have smooth control over the speed like he/she would on a thyristor controlled machine with the result that a lurching or jumping type of "gear change" exists. Only well trained and experienced operators can really get the best out of machines fitted with this system.