There have been some pretty weird definitions of 'internal' from various instructors across the years.
A lot of needless debate has been carried out.

Whole-body soft movement is quite unique and easy to identify.
If you have received an internal strike, you are unlikely to confuse it with anything else.
Anyone who has tried to grapple with a real kung fu person tends to be amazed by their malleability.
The fluid, adaptive approach creates a sense of 'fighting with yourself' or with water.

What people find most odd about kung fu is the ease of the art.
A small movement produces a disproportionate consequence; and no muscular tension is ever used.
Yet it works.

To accomplish this, the tai chi classics must be adhered to strictly, with no deviation.
Yielding must be your first and last thought at all times.