Gently. With respect. Only as needed. Duh.
The real issue is, cervices are not like the plastic rings or models we learned on. Not at all. Cervices are not uniform, they don't dilate in neat concentric circles and we don't use tiny centimeter rulers to measure them in labor.
But we like to act like it.
We give orderly assessments of the cervix, with lots of numbers: 50% effaced, 2 cm dilated, -3 station. This fits neatly on the chart, but does it really help mothers?
Consider this from Pam England: "This shift from left- to right-brain, from conscious to unconscious, facilitates the surrender required in active labor. Without that shift, wherein mind and ego melt into the background, most women could not stand the intensity of active labor."
-Birthing From Within, p. 181
Now consider what many women in labor are prone to do with the string of numbers that result from a vaginal exam. Labor math. This is almost irresistible and goes something like: if I dilate about a cm an hour, and I'm at 2 cms now, I have 8 hours to go. Except I'm not completely effaced yet, so add 2 more hours for each 25% of effacement, and this is my first baby, so 2 hours of pushing, and that's...12 hours? 12 HOURS?!!! And thus the adrenals are stimulated.
Does that sound like a shift from left- to right-brain, from conscious to unconscious? True, active labor produces brain waves much like dreaming. And you can't do math or read in dreams.
So, I'm trying something new. Instead of announcing a set of numbers when I feel a cervix, I do something else. I describe it. "It's very soft and squishy, opening just like it's supposed to. I'd say it's open about like this." And then I show her with my fingers, a visual, tactile representation of her cervix. No numbers.