Coffin corner is the altitude where Vx and Vy are the same.
It is also where the vario reads zero at best climb speed and full power.
Note that best climb speed will be both Vx and Vy, you will have an aggressively nose high attitude and full power. If you increase speed, you go down because you are not manteining a climb speed and attitude (just as when you pitch down while flying at a sensible altitude). If you increase your angle of attack, you simply stall.