Week four of Christy Turlington Burns' Apple Watch diary finds the supermodel in California, where she reveals a few more tidbits about the device's user interface and how it keeps her motivated.

A pseudo Apple Watch brand ambassador, Turlington Burns has been posting regularly to a diary on Apple's website as she trains for the upcoming London Marathon. In her most recent entry, published earlier this week, she provided insight into Watch's tentpole messaging features.

Apparently, Apple furnished Turlington Burns's family with multiple Apple Watches to test out communications functionality. During a spring break trip last week, Turlington Burns and her daughter swapped animated emojis with her husband and son, who were in another area of California.

Interestingly, users can dynamically change the color of smiley face emoji characters using Apple Watch's Force Touch feature.

"The faces are the best because they're so expressive," she writes. "If you press the screen, you can change the color."

By positioning electrodes around the screen, Apple Watch is capable of distinguishing presses from taps. Combined with a capacitive display, Watch can pinpoint where and how a finger is touching the screen, opening up a slew of new contextual UI controls.

Turlington Burns also said Apple Watch Activity rings — Move, Exercise and Stand — validate her philosophy that exercise doesn't necessarily require a visit to the gym. She also expressed how important the device has become as a running partner, especially in terms of progress tracking. Users can offload activity data to their iPhone to examine specific metrics like pace, distance, time and more.

In an accompanying infographic Apple briefly outlines Apple Watch heart rate monitoring, saying, "By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times your heart beats each minute during your workout. That measurement is your heart rate."