There are some really big threads on this board about Diamond Way and lama Ole Nydahl, you can find a lot about it there. But let's try to give a short answer about the practice:
In the DW centers the group will practice the Guru Yoga on the 16th Karmapa, followed by a short invocation of Mahakala. When people start doing a personal practice, it usually starts with the ngondro (prostrations, Dorje Sempa, mandala offerings, and guru yoga on the lineage teachers). After that, almost everyone will do the Guru yoga on the 8th Karmapa. Many will do that practice for life. This is indeed the main path in DW.
Other than those, people will do the practices of Chenrezig, and sometimes Medicine Buddha (Sangye Menla). Furthermore, lama Ole teaches the practice of Phowa in short retreat courses. As a preparation students will practice on Buddha Amithaba. Finally, indeed, there is a practice by the 15th Karmapa. It's a Clear Light practice for awareness during the sleeping state.
You mention the 2nd Karmapa, but the Guru Yoga practice on him (Karma Pakshi) is not being taught in Diamondway Centers. Also, you will not find many of the regular practices that are given in other Karma Kagyu centers. Examples include Shamata, Vipassana, Lojong, and the practices on Green Tara, Milarepa, and all the higher tantric practices like those related to Vajrayogini, Korlo Demchok, Gyalwa Gyamtso, etc.
All practices in the DW are translated, and without extensive prayers etc. Very concise and easy to understand. Not very traditional, but legit. The Guru Yoga practices are possibly the smallest direct path to enlightenment. If you are open to them, it's very powerful. Nothing else is needed. The DW centers are there for those who benefit from this approach. It's a way to have everybody practice the same thing. Need something else? Well, find another place. Again, it's limited, but authentic and complete in itself.
As Magnus has very properly pointed out in another thread, you should always make a choice based on the teacher, not the group. Do you like Ole? Go for it! Don't you like him? Find someone else. Easy
All the best,