With reference to the presentation pp 76, where SSME's Augmented Spark Igniter is said to have their flame "On" even after the main combustion chamber ignition, is to prevent the blowback from the combustion chamber
The igniters turn off after 4.4 seconds while the ignition flame continues in order to prevent intermittent and possibly damaging blowback from the main combustion area. This also keeps the system simple and reliable.
Reference to this video, time stamp 1:08, one can find the following plot
Its clear that the igniter chamber pressure touches 200 psi. Then main chamber ignition happens at around T+1s. It appears like they are shutting down their torch igniter and the subsequent pressure rise at the igniter chamber is basically the blowback from the combustor itself
How are they [Launcher] managing to avoid/withstand the blowback from the combustor as mentioned in the SSME presentation?