All thermoforming techniques are stretching processes. The stretching of a flat sheet of plastic material into a larger shape results in a larger surface area and therefore a reduction in the sheet's thickness. Unfortunately, the thinning down of the sheet is not uniform. Typical thinning is shown below for Female (left) mold and for male (right) mold.

In both cases, that portion of the sheet that first contacts the relatively cool die stretches and thins out the least. This is due primarily to the fact that the material that contacts the die first begins to cool and becomes stronger and therefore resists further stretching and thinning out.
The biggest challenge of thermoforming is to eliminate or minimize the variations in finished part wall thickness.

The first step in a consistent wall thickness is proper mold layout.  The Draw Ratio must be studied closely.  This is the ratio of the vertical size of the mold compared to the horizontal size of the mold.  The ideal ratio is 1:1.  That is, the mold is as wide as it is tall.  Or as wide as it is deep.

Another technique is to pre-stretch the material.  In the next example a box is positioned above or below the mold.  The sheet of plastic is heated to forming temperature, then vacuum is applied to the box.  This pulls the sheet into the box in a nicely balanced bubble.  The vacuum is then turned off, the mold moves into the sheet, and vacuum is applied to the mold, drawing the pre-stretched sheet against the mold surface.  Additional pressure can be introduced into the box to enhance forming detail.  This technique works the same whether the mold is moving up from the bottom or down from the top.