To safely and successfully lengthen the chest, retract the shoulder blades while lowering the weight. In addition to doing so, squeeze (contract) the back and rear shoulders into the bench. Not only does this allow for full lengthening of the chest correctly but it also ensures to keep the shoulders healthy and pain free.
A common action I have seen people perform while training the chest press movement is rotation, not retraction at the shoulder. This does not lengthen the chest. All this does is place unwanted tension and tear on the shoulder. This does nothing for the pectorals, and severely puts the health of the shoulders in jeopardy.