Hair Stacking and Other Applicable Stuff
When stacking any ruminant animal's hair, first remove the underfur as well as the shorter and smaller diameter hairs, for which I prefer to use a fine tooth comb. Make an initial stack. After the initial stack, remove the clump and hold it by the hair tips. Remove any remaining shorter and smaller diameter hairs that did not stack. Smaller diameter hair usually does not stack because it is physically lighter in weight than adjacent larger diameter hair, even when the hairs happen to be of similar length. Now trim the root ends to a uniform length of at least half again as long as you will ultimately need. Check for and remove damaged hair (i.e., broken tips, blood damage, etc.)
A problem may be encountered when removing unwanted hairs from the clump by their tips. Attempting to slide them out of the now even stack causes the scales to catch adjacent hairs' which unstacks a portion of the clump. When feasible always remove unwanted hairs by the cut ends so that the direction of the hair scales do not interplay with the hairs not being removed. This is appropriate when it is discovered that the clump is too large and needs to be reduced. Obviously this approach is not practical when only removing specific hairs such as those with broken tips. The only solution then is to be careful and remove only one or at most two hairs at a time. The more removed at once, the more likely the stacked clump will become uneven.
Next with hairs that are now almost uniform in diameter and weight as well as length, with a more gentle hand restack the hair at 45 degrees or less; less being preferable. Move the stacker to a horizontal position without rotating the barrel of the stacker. Rotating it will roll the now carefully aligned hairs and very likely misalign them. Now slide the bottom tube away from the top tube. At this point the hair is essentially even. If it is not even enough, remove the clump, trim the root ends evenly one more time (do not trim too short for the fly being tied) and make a final, gentle stack. Now you should have very even hair, but it may be somewhat fanned out across the lower curve of the inside of the top tube.
To remove the stacked clump, first, if necessary, carefully gather the hair into a more compact bundle. If removed as a loose bundle the clump will be fanned to a point that it is difficult to work with. Examples of flies that may require such an application include the Royal Wulff, Elk Hair Caddis, and Comparadun. Then again there are some flies that in application require the hair to be broadly fanned side to side, such as with hair "stacked" on top or perhaps beneath a deer hair bass bug. (Here "stacking" has reference to overlaying hair clumps of different colors on top of one another in order to achieve a banding of colors different above from those below such as with a bass bug frog or the head of a Whitlock Sculpin.) Here the fanned clump in the stacker will be removed as it is to retain the splayed arrangement of the hairs. This "stacked" hair on the bass bug is applied over a wide area where that with flies such as the Elk Hair Caddis, etc. the hair is applied over a narrow area. Be sure to pull the clump out of the stacker with the hand that will ultimately hold the hair when it is tied in. Any switching of hands is likely to unstack the clump.