The Tessellation Pipeline - Practical Rendering and Computation with Direct3D 11 - page 284

4.4.4 LOD Transitions and the Risk of Popping
Figure 4.19 in the previous section provides a clear view of one potentially big problem
regarding the transition between different LODs. For both integer and pow2 partitioning,
there is a sudden jump where new geometry is added to the final output. In any animated
rendering sequence where the LOD is dynamic, this sudden change can be very noticeable
to the viewer and generally looks very bad — it is often referred to as popping.
Both fractional_odd and fractional_even have continuous morphing between
levels, which greatly reduces this problem. The changing shape of the surface will still be
visible, but it will be much less noticeable or irritating. However, it is still not completely
immune to displaying the transition between tessellation levels. When the raw input factors
are changing in a smooth and continuous manner, the partitioning will handle it well, but if
the raw factors jump suddenly, the partitioning will similarly jump, and you'll get a "pop"
in the final output.
Temporal considerations are important, here, as this visual artifact is almost always
caused by inputs changing through time and across multiple frames in an animation. It is
important to think about the selection of tessellation factors with regard to how they change
through time, rather than as a single, isolated per-frame factor.