If the simulation is started with the background thread (command bg_run), the .control section commands are executed immediately after bg_run has been given, i.e. typically before the simulation has finished. This often is not very useful because you want to evaluate the simulation results. If the predefined variable controlswait is set in .spiceinit or spice.rc, the command execution is delayed until the background thread has returned (aka the simulation has finished). If set controlswait is given inside of the .control section, only the commands following this statement are delayed.