It works on pretty much all cars with SLOW-0x00.
You just have to turn ignition on, then start logging, then start engine.
That was my process, but every time I started the engine, logging would cease. My issue actually seems to have been cable related. After the cluster switch modification I was able to log after the engine started, but for only ~30 seconds. I tried 3 different 'FTDI' cables (knock offs I'm sure) with similar results. I switched to a CH340 based cable and have successfully logged more than an hours worth without issue.
If you want to start logging with the engine running, you have to apply a small patch to the bin, so it allows "upgrading" the session to KWP2000 when the engine is running.
Is this 'small patch' present in a stock 551M file? I presume it is since I can initiate a log with the engine running.