>>361840150
IK tweaks allows your spine-chest joint to bend among other things. It means if you're slouching irl your avatar will also slouch. Without it your back is perfectly straight all the time so your irl head will be lower than your avatar's head when you're slouching. Solutions are either IK tweaks or better posture.
I use IK tweaks specifically because it lets my torso rotate when lying down. Means I can actually lie on my side instead of every part of my body being sideways but my back being flat on the ground.