Shadowboxing like you said flows and is done spontaneously from techniques you train often. I have always compared shadowboxing to other more resistant training because you are going through the motions at a slower pace but with some intent still behind them.
You can pick apart basics and forms and do those techniques in shadowboxing, as you should. Everything form punches, kicks, knees, elbows, animal techniques, and throws can be practiced and simulated in shadowboxing. I find this more beneficial than forms because forms being a set pattern does not provide you with different scenirios, it is the same thing over and over and although people may say it can be used for this or that, more often it is made to fit in the mold of this or that when someone tries to explain something.
The whole idea of techniques hidden in forms or forms having secret moves is a load of sh**it. Take the techniques from the form, than practice how you would use them sparring or fighting, and shadowboxing builds a good base for this.
"The hero and the coward both feel the same thing, but the hero projects his fear onto his opponent while the coward runs. 'Fear'. It's the same thing, but it's what you do with it that matters". -Cus D'Amato