This trains the principle of connecting the spirit and intent, and the principle of crossing the arms to gain control. Since bagua uses stepping, one of the main defensive techniques is to step a bit aside and cross the attacking arm with your arm, setting up a press down or a drive in (much as you would do with a spear).
Partners stand facing each other at a sparring distance. Then each steps into chuanzhang on the same side, attacking the partner's throat, so immediately connecting with the partner's arm. Do proper triangulation steps to develop clean footwork. Step calmly like a tiger – do not bob and weave. Pause slightly when in contact, then step back away, setting up the hand near the body, so that in your mind the technique is already done. Use your spirit to stay connected.
Think of the chuanzhang as a setup for a following technique (if the chuanzhang gets in you are setup for a zhuangzhang, for example). This will prevent you from getting too heavy with it.
Daishou can connect to the chuanzhang partner practice. If your partner comes in strongly, then continue the movement by pulling.
Rotating chuanzhang can connect to the chuanzhang practice. If your partner feels weak then move in and stab again.
Video of Andrea and John doing chuanzhang partner drill, moving on into single palm change and then single hook.