From BlenderWiki

Push/Pull

This transformation consists in translating all selected elements along the line joining their original position to the pivot point. All translations are of same value, and controlled by the mouse. It gives something that reminds a bit the scale effect, but much more deforming.

Note that unlike the preceding ones, you can lock this transformation on axis – even though this has no real interest (except perhaps with a “plane locking”…).

Its hotkey, ⇧ ShiftP, only works in Edit mode (as it is already used by the “preview” feature, in Object mode…).

Example

TODO: add a short example…