I'm writing these instructions because the section of the game where you're trying to save two strung-up guards from being electrocuted by Harley's trap is VERY FRUSTRATING if you don't know how to use the sequencer properly, and if you're anything like me, you really tried to figure this stupid thing out yourself (no instructions whatsoever, nice), and IMO what other people have written as a guide for this bit of kit boils down to “hold down A for a while, then try the left mouse button... err, it may be more complicated than that... try holding down three things at once... it's a bit tricky”, yeah. Some people may have preferred to find this information back when the game was released, but I've only just started playing it
Assuming you're using the default keyboard and mouse controls, and you're right-handed, then your left hand is near the A/D keys, and your right hand is on the mouse, which roughly corresponds to Batman's hands on the sequencer. If you're left-handed, then you'll just have to accept that your mouse is on the wrong side of the keyboard
To demonstrate the interface:
Press the A key once, and the left control of the sequencer will move a little anti-clockwise. Press the D key once and the left control of the sequencer will move a little clockwise. Got the hang of that? Good.
If you hold down say the A key enough to make his finger do one full circle on the control, and the sequencer hasn't made any interesting noises, then there's no point in taking it any further. If it has made any interesting noises, then backtrack a little with the D key until you've got the control making interesting noises (or the edges of the graph are much closer to the edges of the screen). If you've cracked it already, the screen will go green and the sound will change considerably, then simply not pressing any keys will cause the system that Batman is trying to hack to explode (within a few seconds), and you've succeeded. If not, move on to the left mouse button and do the same routine you just did with the A/D keys.
If you're trying to rescue the two guards in the penitentiary strung up by Harley Quinn, then you have to do the control panel just to the left of the window Harley is at, then batarang the first guard's rope, then run around the corner to the left, into the water to the second control panel, then batarang the second guard's rope, then run back to the door you originally came through and hack the panel there in ten seconds. Each one is tougher than the last, but once you've got the hang of the interface, well, I did the last one in about 5 seconds. When you're done with that, run through the door (or is that a cut sequence, I can't remember). You can also save yourself a bit of time by remembering what control yielded the biggest result first for each control panel.