Well, imagine that the splitshaft is a clutch that connects the the front axle into the rear axle (with it being constantly connected to the transmission). Usually it locks automatically depending of the situation. For example, the Bastion in Savage mode, has a minimum lock coefficient of 0.25, which means that the clutch is at least 25% engaged. When it reaches 100%, it behaves like a locked center diff. Usually it fully disengages when the TPS is 0, so, if your rear wheels are stuck, you have to give at least some throttle for the clutch to engage the minimum amount. With that system, the front wheels are only connected to the rear axle when you have some throttle in, so handbrake turns are possible. That is true for the Clutched splitshafts. Viscous splitshafts work differently.