Shadow combos are a funny one, its basically shadow boxing but a tiny weeny bit different, with shadow boxing you practice your fluidity of movement, the speed of your punches and kicks as well as defensive tactics and offensive tactics.
With shadow combos you just pick a combination you want to practice and throw it out slowly, deliberately and with bad intentions. Each strike you throw should feel powerful, there should be almost no speed, or at least, not as much speed you would normally use.
The whole purpose of this drill is to build your mind muscle connection, muscular control, as well as cementing those neural pathways, so that when you throw the techniques you need, you’ll throw it almost bloody perfectly. Some of my favourite techniques to practice with this drill are jab + cross + lead uppercut, switch kick + lead knee + lead elbow and jab + rear elbow + hook.