Why it works
The angle opens the chest cavity from behind, giving a long path through the vitals with little bone in the way. Many guides prefer it to broadside.
How steep is too steep
If the entry point moves back past the last rib, the bullet travels through the paunch first. Wait for a gentler angle.
Typical miss
Aiming at the near shoulder as if broadside, which sends the bullet out in front of the far lung.