Loyalty
When a follower's cost is paid, they hold +1 Loyalty, up to 3. If a follower holds 3 Loyalty, then Strengthen Your Bond doesn't trigger.
The follower holds the Loyalty but the PC they follow (or rather, that PC's player) chooses to spend it. As a rule, a follower is tied to one PC, and only that PC can pay its cost or spend its Loyalty. But if an NPC follows the party as a whole, then any PC can generate or spend that follower's Loyalty.
A PC will typically spend a follower's Loyalty after you make a GM move and show that the follower is afraid, or about to do something dumb, or doesn't want to do as they're told. The player can spend Loyalty to resolve that problem without having to trigger a move like Persuade or Defy Danger.
A player can proactively spend Loyalty, too, but if they do, consider whether it's really necessary. If the follower wasn't going to balk or do something dumb, tell the player that there's no need to spend the Loyalty after all.
It's easy to think of a follower's cost and Loyalty as transactional: I did this for you, now you owe me. That's true for some followers, but for most, Loyalty represents trust, respect, fondness, or even just habit. Ask the player what it looks like when they spend a follower's Loyalty. Maybe the PC claps them on the shoulder and says something inspiring. Maybe they make eye contact and shake their head. Or maybe the follower just swallows hard and does what needs to be done. Work with the player to establish how this particular follower's Loyalty manifests.
The trip to Gordin's Delve takes four days, and the first day we establish that Blodwen is caring for Fflur. She's taking pains to groom her in the evening, brushing her down and keeping her company while she grazes. We agree that this pays Fflur's cost ("extensive care, grooming") and Blodwen's player marks 1 Loyalty for Fflur.
The next day of travel passes without any significant scenes. Blodwen takes care of Fflur again when they make camp, and asks if she Strengthens Their Bond again. "No," I say, "you just did, right? Like, yeah, a day passed in-game but you've basically been caring for her this whole time, right?" So Fflur still holds just 1 Loyalty.
The next day, a frythanc (a huge bird of prey) swoops low over the party. The magic of the roads will keep them safe if they stay put, but I show a downside and play up Fflur's instinct ("to panic"). "Blodwen, Fflur bucks, rears, and knocks you back, tumbling onto the pavestones. She's still in the tackle, and if she keeps this up, she'll, like, break her leg. What do you do?"
"I get up and calmly approach, hands out like 'whoa, girl." She's trying to use her Into the Lion's Den move, which lets her calm beasts with a touch.
"I know the move says she won't harm you, but she's still kicking and rearing. I think there's still a danger that you flinch or show fear. You'll be Defying Danger with WIS. Or you could spend Fflur's Loyalty to keep her from acting on her instinct."
Blodwen uses the Loyalty and I ask what this looks like. "I'm like whispering to her, hands out, 'shhh girl…" and she calms down. Does that work?" It sure does!
Things are getting creepy out in the woods. The sun just dimmed. Trees are bleeding. Ominous shadows flit just out of sight. Vahid is painting a sanctifying mark on Caradoc's shield, but it'll take time.
"While they do that, Rhianna, your crew is kind of freaking out. What do you do?" She whistles and directs them to encircle the PCs, intending to have them Defend.
"Yeah, but I think they're Defying Danger first," I say, "the danger being their own fear. You know how superstitious they are!"
"Uh, no," says Rhianna. "I'll spend their 1 Loyalty to overcome their fear to do as I say. I don't think it looks like much, that whistle just snaps them out of it. So… I roll for Defend, then?" She sure does!
Gwendyl hasn't shown up for Vahid's Breaking-of-Fast party, and Blodwen goes to her house to find out why. Gwendyl claims she doesn't hold with "those decadent southern ways," but really, she's feeling neglected by her favorite pupil.
I expect Blodwen to press Gwendyl and trigger Persuade. Instead, Blodwen gives her a hug, apologizes for not being around, and says, "We don't need to go. Plenty to be done here." They spend a few hours cutting and bundling herbs. Then Blodwen asks if this pays Gwendyl's cost of "consideration, affection." I sure think it does.
"You know, there might be some baklava left at Vahid's," says Blodwen. "I spend that Loyalty to get her to do something she doesn't want to do." But, now that she's feeling better, I think Gwendyl would be fine going to the party, and I tell Blodwen to hold onto that Loyalty.