You can call it delivery, you can call it swag, confidence, presence... but basically it all comes down to having a powerful vocal performance, no matter what area of Hip Hop you function in, its a crucial component to your style... and alot of of rappers go without learning how to use it properly...
A Strong Vocal Performance consists of 4 Parts:
1) Energy, This ones obvious, you cant sound like your tired, you have to be focused and in the zone..
2) Vocal Projection, Speaking loud without yelling.. Using your diaphragm to push as much air through your vocal chords as possible.
3) Tonality, The Context in which you deliver your words, for example saying something in a sarcastic tone, communicates something much differently than an angry tone, the tonality changes the perceived meaning of the words your using.
4) Resonance, This is probably the hardest one but the most effective... Resonance is what happens when, your throat muscles are relaxed and space is created in your throat and nasal cavity so that the vibrations from your vocal chords have more space.. This creates that nasally but really interesting sound rappers like lil wayne and jadakiss are known for...
What NOT to do:
Dont YELL.. all yelling does is strain your vocal chords... the sound of yelling is actually very off putting to the human ear... you may not notice but the sound will always be off..
Dont speak from the throat.. Everything comes from the gut.. that easier said than done, but if you want the most out of your voice you definitely want to practice speaking from ur diaphragm..
I suggest taking singing lessons, just explain to the instructor how your just trying to improve your resonance and tonality and he will probably be able to show you some exercises you can use to strengthen vocal muscles and relax the larynx.