For the very curious!
Original Leslie Brake Accessory
When two-speed Leslie speakers such as the 122 and 147 were originally introduced, they included a provision to allow the addition of an optional "Brake Accessory". This Brake Accessory was actually a small sub-chassis containing a delay relay that mounted on top of the Leslie amplifier. Attached to the Brake Accessory's sub-chassis was a separate cable connected directly to the tremolo switch at the organ. When the tremolo switch was moved to the off position, the delay relay would activate the slow motors for 5 seconds to slow down the heavy wooden rotor. After the 5 seconds elapsed, power was removed from all of the motors. While these units did in fact allow the organist to stop the Leslie rotors, they were never all that popular. The extra cable and expense certainly didn't help.
EIS-122 and EIS-147 Relays
During the development of the EIS-122B and EIS-147B relays it was realized that several additional features, including the function of the original Leslie Brake Accessory could be added without actually increasing the cost. In addition, the relays could be designed so that there would be no need for a separate cable just for the brake. On EIS equipped Leslies, no alterations at all are required at the Leslie to implement fast / slow / stop operation. An appropriate switch at the organ wired with a supplied diode is all that is required.
New (Rev 5.0 and later) EIS-122 and EIS-147 Relays
While a significant improvement over the original Leslie Brake Accessory, the original EIS relays still provided a fixed braking time. This duration had to be long enough to adequately slow down the rotors from their highest attainable speed. The only drawback to this approach is that the Leslie rotors don't always happen to be spinning at their highest attainable speed when the organist decides to switch to stop.
If switched to stop before the Leslie rotor had achieved full tremolo speed, the old brake systems would still provide the full 3 - 5 seconds of slow speed motor operation before finally allowing the rotors stop. Many musicians asked if this annoying delay couldn't be eliminated. In order to eliminate this delay, the braking system would somehow have to be aware of the current rotor speed. This is exactly what the new EIS relays do.
The new EIS relays monitor the amount of time that the Leslie has been running at fast speed. Then, using an accurate software model of the mechanical rotor system, the relay is able to calculate the current rotor speed as well as the optimum braking duration required for that speed. The end result is no more waiting for the full brake cycle to time out when it isn't required.
