Originally Posted by
Ti-tillIdie
why does it say that it's safer for the rear rider to have a brake too
Looking at the photo of the beast, it would appear as though the stoker has control over the somewhat smallish diameter rear disc, whereas the captain is controlling the rim brakes on the front and rear wheels... most likely with both front brakes being controlled off the same lever (just a guess).
Assuming the inventor based some of his bike's features on what he's observed on other tandems, the stoker's control over the rear disc is used to provide the tandem with a drag brake to continuously scrub off speed on long or steep and challenging descents, while the captain uses the rim brakes to slow the tandem as they enter turns, etc... This same configuration is not all that uncommon on tandems, except that an Arai drum brake is usually used as the drag brake given it's incredibly high heat tolerance.
As for how the stoker knows when and how much to apply, initially it takes a lot of communication with the captain, e.g., give me about 50% on the drag brake sweetie! For teams that use a stoker controlled drag brake on a frequent basis, over time the stoker and captain will just work out their technique where using the system doesn't take a lot of communication: the stoker will just know instinctively when it will need to be used or know just how hard to apply it when the captain calls out for some drag brake control. This is a actually not a bad way to enhance the whole concept of 'team work' on a tandem.
Frankly, although most stokers may not realize it, if the rear brake's control cable runs down the side of the top tube they already have the ability to control the rear brake.... just reach down and pull up on the cable.