So I was wondering how people managed to kick out Ukes lumbar and only his lumbar and his chest. Every time I seem to try it Uke just splits in half like a normal split. I have been wondering this for a while now, thank you in advance.

An example is posted below

Pause it right before the hit and advance frame by frame.
At frame 193 you'll notice that his foot is inside his chest while not yet breaking it. Ghosting your foot/hand into bodyparts at high speeds will spread the damage out around a large area of joints while damaging all of them.
Momentum, keeping your flow, aim and building lots of speed is essential to doing this. Getting your foot inside uke's body takes practice.