The staggered build/index mods use a time value for how long the process should run before pausing, i.e.; run process for x seconds, then pause, then run process again for x seconds, the pause, etc. until the build/index is completed.
This is invaluable if you're on a shared/virtual server and your ISP has limited the amount of time you can run a single process for, as it stops the server 'timing-out'.
There is a variable in the Links.pm file that sets the value for how many seconds the process should run in each cycle so you can tune it to get the maximum 'run' per cycle.
Hope that helps.
All the best
Shaun