Squeeze your Chest! Straighten your back and also keep it more stable in the negative.I'll remind the chest up cue as rippetoe tells it.
When squeezing your chest at the bottom it must not feel easy or you are doing it wrong.
By squeezing your chest the spinal erectors from the top to the sacrum should be contracted as a chain straightening your back while you are feeling a mild stretch to your hamstrings and then you pull......and you are filled with happines joy and strength!