resonance is something you don't even really broach until you're already quite advanced, because it's not as if there's one technique or body part you can use and say, 'now i'm doing resonance.' it's more about getting to know your body in a fine-tuned and yet holistic way.
(on a final note, i'm thinking of doing these analyses in tiktok form but i'd feel awkward doing it, lol. but if you'd like to hear me talk through this stuff with audio demonstrations, lmk! i'm not sure what i'd use as a visual in the videos. lmk if you have any ideas, haha.)
resonance gives you a fuller, more layered, more magnetic and more interesting sound. it's why some singers just have 'it'. (the first singer that comes to mind when i think of resonance is adele.)
now let's talk resonance. ohohohoho, resonance. resonance basically means all the little frequencies in your body and the space you're in that bounce off the structures around them and interact with the main note you're singing.
(i'll talk in-depth about larynx placement during sunoo's voice analysis, but for now suffice to say that lowering your larynx is another sure way to get relaxed.)
there are many little hacks to get relaxed, so here's a hack i use. if you're approaching a high note, or if you've just finished a series of high notes, you can silently use the onset you would use for a low note, or make a sob shape with a low placement to lower your larynx.
a relaxed voice sounds good. it sounds free. heeseung doesn't use excessive vibrato, but when his natural vibrato does come through, it just shows how relaxed his voice is.
since we're talking about heeseung i should talk about vibrato. vibrato is something you can do deliberately, but it also happens naturally when your voice is very relaxed. relaxed is how we want the voice to be, which is why so many techniques are about removing tension
this adds a more interesting texture to what they're singing. think of how an idol sings to a fan on a video call, or a teen movie where the characters are showing off and trying to woo each other. there's something showy and almost romantic about mini slides.
onto slides, particularly mini slides. to aid their smooth sound, a lot of rnb singers (and even country singers) add tiny pitch slides up or down into each target note as they sing. the end result is a very smooth, almost 'easy' sound, rather than singing each note 'straight'.
most runs and ad libs are based on the scale of the key you're in - e.g. heeseung's iconic descending run in bills is just the pentatonic scale. ily pentatonic scale. it's worth practising scales for many reasons, but one of the more fun benefits is that it'll help with runs.
(about the word 'soft': if i mention a 'good' soft i'm talking about head/mix/thin chest, not breathy onset. breathy onset also sounds soft but it can dry out the voice a lot unless you know what you're doing. i used to do this a lot, and that shit hurts. but back to melisma...)
since we've broached the topic of rnb, let's segue into slides and melisma (the technical term for runs). melisma is fun as heck and it's easiest to have the flexibility to do it when your voice is kept light and soft, like in head voice.
what heeseung's actually doing is he's pulling his head voice down into his chest voice to get that mixy sob. i.e., even when he's singing in chest, he's injecting a bit of head voice into it. that's what gives it that smooth, almost effortlessly sweet sound. it's popular in rnb.