When one or more objects press into the breast, generally with the majority of its force being directed towards the center of the breast.  The penetrating force doesn't break the skin, or otherwise breach the "skin boundary".

Some examples:

* A typical breast poke: post #100028.
* Not a breast poke: post #198905.
* This is a breast poke: post #234777.

Where you may call the second image a breast poke, it is instead [[nipple penetration]] because it is breaching the "skin boundary".