In layman terms working set is used to predict locality of a process, so that we can predict how many and what pages a process may be needed for execution. If we do this we can reduce no of page faults and hence thrashing can be reduced.
Wss i = working set required by an i_th process
∑ Wss i = sum of the sizes of the working-sets
D = total number of available frames.
Case 1 : D > ∑ Wss i
Before the start of referring pages for a process, with the help of pre-paging we bring all the pages required by a process into the available frames. We can do this for every process because we have enough frames since D > ∑ Wss i .
"It initiates another process if there are enough extra frames". This line means we can pre-fetch the pages required for a process into available frames and start its execution.If we do this chance of thrashing are very less because very less page faults may occur.
Case 2 : D < ∑ Wss i
We have less total frames than total pages required by all processes, then the chance of page fault is very high.To prevent page fault we suspend a victim process ( i.e move it secondary memory). When we suspend the process then '∑ Wss i' term decreases then chances of page fault decrease, hence chance of thrashing decreases