That static feeling isn't the power Reiner knows, carrying with it no taste of ozone. However, it's noticeable enough that he assumes Hakkai is using some sort of ability. (It's a testament to how comfortable he is with Hakkai that Reiner doesn't tense.)

Then Hakkai unleashes that unknown power, slamming it into the kelpie's chest. Reiner has to force himself not to stop and watch. He's not a spectator here; he's Hakkai's quest partner.

Deciding to hold off on transforming until they try other methods, Reiner drops to one knee and presses a hand to the ground. Doing so isn't strictly necessary for what he does next, but it helps him focus, making magic seem more manageable.

The ground underneath the three unharmed kelpies abruptly drops, the kelpies sinking into a rapidly forming pit with sheer sides. Reiner is trying to pen them in. He hopes to secure at least one before they realize what's happening and try to leap out.