Master Yang didn't train the 18 Luohan with his teacher. He doesn't teach anything that he didn't "master" by traditional standards. Still, he has researched and trained them himself.
According to Master Yang, the 18 Luohan is the same as the Muscle/Tendon Changing qigong. The theory is the same, the movements evolved over time.
I have also looked into it. Here's an excerpt from an article Im working on:
Both Bodhidharma and the Buddha arose from the Indian warrior class, known as the Kshatriya caste. Their martial arts were called Vajramukti, a name meaning "Thunderbolt clasped hands". Based mainly on barehanded combat, Vajramukti was practiced in peacetime as physical training for health. The postures or forms were known as "pratima" and were linked together into sequences, known as "nata", which were meant to train both the mind and body. The most important of these sequences was the 18 Subduings (Astadasajacan) which reflect the Buddhist doctrine of the 18 perfections, or paramitas. In China, this sequence became known as the 18 Arhats or 18 Lohan, which means "perfected person", and refers to enlightened disciples of the Buddha. Bodhidharma translated and taught this Mahāyāna Buddhist yoga, documented in his Muscle/Tendon Changing Classic, or Yi Jin jing. Bodhidharma also translated and taught Buddhist therapeutic kriya yoga (Asthimajja Parisuddhi) for "Bone marrow cleansing and purification", known as Xi Sui Jing in Chinese.