Tori and uke place a jo (staff) between their centres and take it in turns pushing each other slowly and strongly across the mat. The aim is to become aware of initiating strong movement from the centre while maintaining balance and posture. If you find it painful to hold/push the jo strongly, put a book against your belly to dissipate the force of the push thereby allowing you push even more strongly. Also, if one partner seems much stronger than the other, give the weaker one the book and see how much they instantly improve!
When moving onto technique, say ikkyo, draw your attention to the jo exercise - do the technique with the same kind of push-feeling, as though pushing uke from your centre rather than through the arms. Likewise, uke should coordinate his attack by moving centre and striking arm in unison.