Here, the experience from the external is undergone with no sensory input. For instance, the sound of a guitar does not reach the field of sensory experience that is "sadda". One is "insensitive" to the sound, so to speak. One is neither attracted, nor repelled by the sound.
As far as the internal experience is concerned, one focuses on the body (breath and body).
First, because this is the first occurence of a saṅkhāra in paṭiccasamuppāda; secondly, because the body is the most "controllable", (through ānāpānasati, for instance,) and the most "personal" part of the internal.
One focuses on the internal body.
(Yet a body made of external khandhas - something to be transcended at death).