Netherlings have magic birthmarks that look like a little maze somewhere on their bodies. Placement of the marking differs between netherlings. It can be used to unlock stuff and for Melding.
Aperently touching birthmarks can cause the tingle-tingleEdit
"I grasp his wrist with one hand, tracing the lines with my other... his features shift and a rumble escapes his throat - something between a purr and a growl. his arm tenses, as if it takes his full concentration not to move while i appease my curiousity" (Howard 167). It could just be a Morpheus thing, so Alyssa toughing him would be supper excite. The book mentions an electricity type thing between birthmarks when melding, so maybe tingle-tingle?