For Reiner, it's Porco's vocalisation, any sound he lets out that denotes his pleasure; the way he arches his back and writhes under him; trembles and shivers after an orgasm, and seeks out his touch as he drifts off to sleep.
And in a sense, for Porco's it's similar: he responds to Reiner's low utterances and his strong hands on his hips; to the heavy weight on top of him when Reiner presses him down with his own body; to the tension he senses in Reiner's body that makes his own freeze for the blissful moment in which Reiner reaches his orgasm and then collapses atop him.
These two men as lovers are the definition of body worship.