Yet more horse watching today. 2 mares, lets call them Bree and Sapphire (not shown in the photo below, as they weren't my horses). Sapphire has a sore back, although I don't know the details about this.
Bree and Sapphire are mutual grooming. Bree digs her teeth vigorously into Sapphires mane and works along her neck, Sapphire responding with equal enthusiasm.
When they move on past the withers though, it changes. Sapphire continues to groom 'normally' with her teeth, but Bree shuts her mouth and makes a gentle circular motion with her nose until she is almost level with Sapphire's hip. She then resumes vigorous grooming with the teeth. Wish I could have shared it on video here, quite special.