Research into Topology
Origin
The Definition of Topology and Its Purpose in 3D Animation
https://www.lifewire.com/topology-in-3d-animation-2181
Think of 3D topology as the wireframe of an object. Topology refers to the geometric surface characteristics of a 3D object.
In computer-generated 3D topology, the goal is to have sufficient detail in the wireframe while keeping the number of faces to the minimum necessary to accomplish the job.
3D modelers strive for "clean" topology, typically illustrated by a 3D mesh with efficient polygon distribution, proper placement of edge-loops, few or no triangular faces (as opposed to four-sided "quads"), and clean precise creases that minimize stretching and distortion.
Topology
http://wiki.polycount.com/wiki/Topology
Topology is the layout of a model, how the vertices and edges are placed to create the mesh surface.
Good topology is essential if you want fast framerates (realtime) and good deformation (both realtime and pre-rendered). For realtime rendering, bad topology can also create rendering problems.
In-game models should be optimized to create a good silhouette, define edge-loops for better deformation, minimize extreme changes between VertexNormals for better shading, and allow for good UV seams.
Vertices and edges need to be in certain places to give good bending/compressing/stretching when the model is skinned or morphed. Topology is the most important where the model has to deform the most: crotch/hips/butt, shoulders/armpits, mouth corners/cheeks, knees, elbows, hands/fingers.
Avoid T-vertices, doubled faces, gaps, flipped faces, internal unseen faces, floating vertices, etc.
Add more polygons where there's curvature, less where it's straight.
If a shape is used that isn't a quad when modelling, it will distort or more likely, disappear when added into the game engine.
When a model is added into a game, 'Triangulation' occurs. Every quad it finds, it cuts in half to turn it into triangles. But it can't do this with any other shapes, only Quads. With a different shape, it will keep cutting it until it gets a triangle, which won't happen, so it makes the shape disappear.
When smoothing a model, the program will add more topology using tesselation. For this reason, edge loops need to be all the way round.
There are some tools in Maya that can help fix topology - clean up and triangulator / de-triangulator.