On the 1200/1250LC,
@rico, the linkage is achieved by the ABS control unit applying measured pressure to the rear brake lines when the front lever is squeezed. The level of pressure is determined by the riding dynamics of the moment, the result being pretty much optimum braking for the circumstances - better than most of us mere mortals can achieve. You can observe this with your LC by wheeling the bike with key on but engine off - as you apply the front brake lever, you will hear the ABS pump activate.
This makes for a very neutral stance for the bike when trail braking as the bike is leaned further into a curve, easing off front brake lever pressure as the bike leans further. Normally a bike will tend to stand up somewhat if the front brake is applied in a turn, and conversely a little rear brake will tend to pull the bike in tighter.
Rather than completing all braking before a curve, trail braking with the front brake lever is a useful skill for racers, track riders and brisk road riders as it loads up the front tyre creating more grip and sharpens steering geometry with the weight transfer, allowing faster speeds through a curve. It’s also useful for everyday riding because the front suspension is already compressed and if you find you do need to brake due to road/traffic conditions, there’s no sudden weight transfer and geometry change that could affect traction and stability. Note that rear braking doesn’t create as great a weight transfer compared to front (hence more fork compression with the latter) but then it is less effective and more prone to lockup (and sliding out if you use it too hard in a curve).
Getting back to the thread (😅) in the case of the new 1300 platform, the front brakes are also linked to the rear lever, which as far as I know is a first for BMW bikes. The tester found that in Road riding mode, this linking was markedly noticeable and didn’t suit his riding style - noting that he is a racer, so is more likely to want to control braking balance more than perhaps the average rider. It would also limit the rider’s ability to tighten the bike’s line in a curve by rear brake lever application.