SoYeon's real forte when it comes to singing is her phenomenal vocal control.
Let's start with the first example:
"SoYeon is more about blending in then standing out; rather then purely standing out" -- TimeToT-ara
Notice how when the melody is passed from EunJung to SoYeon at 1m17s, the transition is so smooth that on the first few casual listening, I didn't even notice that the line is passed. And very quickly after the transition, SoYeon's iconic resonant voice penetrates through.
Fast forward to 3m06s for another transition from EunJung to SoYeon, but this time, it's in the bridge and it ends with SoYeon's decaying - as opposed to piercing - high notes. (after all, it's the golden rule of KPop for main vocals to sing high)
This following video may have been over-shared by me, but I find it appropriate to be included here. Do listen to the start point.
also, 1m27s -- SoYeon's rap with a "growling" part
I've been too bias for my bias, should listen out for other members too.