Soft Preemption means that a TCH is kicking out an allocated PDCH.
If you increase MAX PDCH, it means that more PDCH are allocated in the cell. But as soon as the “TCH load” of the cell is too high, the PDCH are released in order to make space to the increasing amount of TCH.
The number of PDCH that cannot be preempted = MAX PDCH HIGH LOAD, even if the TCH load is increasing. Those are non-preemptable PDCH.
To conclude: soft preemption means that your TCH+PDCH are too numerous in the cell. In order to avoid TCH congestion, the PDCH are deallocated.
Another thing: there are two types of preemption : soft and fast. It’s not important to understand the difference, since the management of the preemption is fully automated (no parameter can be tuned). A PDCH is deallocated with soft preeption if it also carries the PACCH of one TBF.
Otherwise, the PDCH is preempted with “fast” preemption.