"simultaneous pecking from inside and outside"
When a chick is struggling to peck its way out of its eggshell, sometimes its efforts alone are not enough. To help her baby come into this world, the hen will gently peck at the shell too, so that by their combined pecking, the baby chicken may escape its eggshell. Furthermore, the chick may one day grow up to be a hen, and peck the outside of an eggshell of her own offspring...and so the cycle continues.
This phrase is used as a metaphor for the interaction of a student and a Zen master. As the student struggles with Zen (like the chick pecking the inside of the shell), the master will try to encourage and direct the student's mind (like the hen pecking from the outside). Once the student has exited zer mind's eggshell, zie may one day grow into a Zen master, and continue the cycle by pecking at the mind-eggshells of those still trapped inside.
The Japanese phrase is translated into Chinese as cuizhuo-tongshi.