In the case of some liquid and viscous samples when the light (in the case of this post-dispersive NIR instrument) cross the samples, we can lose a lot of energy (transmited light which does not reach the detectors)
See this case (in the reallity is not so much light, because the photograph over-exposed the core of light) looking the sample from the top side:
so maybe it become necessary a reflector, to work in transflectance. Video show how the reflector is placed over the sample cup looking from the down side.
When doing this take care that not bubbles or any other non desired material is on the pathlength under the reflection area.
In the case of chocolat it is not necessary to use the reflector: