The lower the knob, the physically larger and more powerful the transformer will be. This translates to more headroom and a fuller dynamic range, because the larger transformer can more easily handle that signal. With the knob set high, the transformer shrinks and becomes less able to handle all the volume you're putting through it, so it will more readily sag and breathe under the stress of the notes, then bloom when the notes decay, because it's weaker and more sensitive to change.
Think of it this way. Your guitar tone is a cube that magically becomes physically heavier as your guitar's signal increases, and lighter as it decreases. The transformer is a wooden plank set across a gap that forms a bridge, suspending the block in the air. As you play, the magic cube will fluctuate between weighing a lot and weight a little, going along with your guitar's output, and the wooden plank will flex from the weight of the cube, relative to how much the cube weighs at the moment. The straighter the plank, the clearer the signal.
In this metaphor, the Transformer Match knob basically controls the thickness of the wood.
Low values make the wood really thick, so even when rapid fluctuations happen in the size of the cube, the bridge stays relatively stiff and steady, and the signal remains at full bandwidth and low distortion no matter how the cube fluctuates. High values of the Transformer Match knob will make the wood very thin, so that the bridge will flex very easily. Even small fluctuations in the size of the cube will be very apparent in the overall sound in this case.
I personally like my tone a little more on the slightly tighter side of things, but not so stiff as to sound lifeless, so I tend to turn the knob down just a bit. Plus, it's really cool to be able to turn one knob and hear your amp's guts get stronger in real time, haha.