What is a threshold concept?

One of Meyer and Land's key characteristics of threshold concepts is that "They are irreversible – once learned, threshold concepts are unlikely to become ‘un-learned’ or forgotten." This reminds me of riding a bicycle and other learned practical skills (difficult when you start and you have to overcome a mental and physical 'blockage' to acquire them particularly when they require counter-intuitive actions e.g. leaning downhill on a snowboard).

Are threshold concepts, then, purely mental things to be acquired or could they also be physical skills (bearing in mind the links between the cognitive, physical and affective domains)?