This is a basic Wing Chun drill called Grabing Hands or Bon Sau Lap Sau.

(1) Start in a left neutral side stance. Your partner starts in a right neutral stance. (2) Step forward with a left vertical punch. Your partner lifts up his right wing arm block to deflect it. (3) Your partner uses his left arm to open a path for a counter useing Lap Sau. (4) Your partner follows up with a vertical punch. You lift up your left elbow to execute a left wing arm blockto deflect the punch. (5) You follow up with a right grabing hand block on your partners right arm to open a path. (6) You execute a left vertical punch. The drill continues in this way.
The foot work is like this neutral stance step into a front stance and punch.
Blocker is in a neutral stance and blocks then uses the grab and steps forward to punch. Person 1 steps back into a neutral stance as the punch comes in and uses the wing arm block and the proces continues on.