Two difficult areas on bows cause even the experienced tillering eye to strain sometimes.
The Buchanan dips is one such area (also known by the more modern term of "fadeout"). The dips are that transition area that graduates movement from static handle toward bending limb. It is the area on the belly side where the handle dips down to the working limb. The dips must be accomplished with much finesse. Make them too long in relation to bow limb length and you place unnecessary strain upon your limbs. Make them too abrupt or dramatic and you risk increasing hand shock or creating a hinge from a concentrated compression point. Make one longer or weaker than another, and the bow will not balance in your bow hand as you draw it.
The other such area centers on the humpy-bumpies that sometimes roller coaster along a stave limb's length. They are usually found in the gnarlier woods, such as osage. Even steamed and corrected staves often contain such troublesome areas, areas that interrupt otherwise smooth limb flow. As a solution, the bowyer is usually counseled to "see" the bend through them, to try ignoring them in favor of the big picture. But it confounds the eye to watch a deflexed section of a limb appear as anything other than a hinge when the bow is drawn to full measure, or to watch a reflexed section turn into a flat and still see it as something other than a stubborn, unyielding blemish upon an otherwise perfect limb.
In both cases, how does the bowyer detect subtlety of movement? How can he see whether or not an area under his concentration is moving the way it should, the way he wants it to? We often know what we want. The problem is recognizing whether we have it or not.
My simple solution to seeing bend in critical areas is to use a reference stick upon them so the eye can study relationships up closely. This way: Brace the bow and place it upon your tillering tree. If you are finessing the dips, clamp a straightedge or a simple piece of wood, say 1x1x16, exactly over the handle and nearly resting upon it. Use a spring clamp. Center the referent so equal amounts project over either side of the dips, and also so the gap between it and the bow's back looks identical on both sides of the handle along its length. Now, with your pull cord attached to the nocking area of your bowstring, stand up close to the handle area and watch what happens when you haul upon the pull cord. Watch the gap change between the bow and the referent straight edge. Haul upon the pull cord repeatedly as you watch the gap come and go to both sides of the handle.
This technique will enable you to see very quickly how far into the handle area movement ceases, how far into the limbs movement becomes full, and whether or not the dip area is synchronized between limbs, i.e., whether or not movement balances out the same amount in the same area from one limb to the other. Simply study and make judgments along the length of the gap created against the referent as you repeatedly flex the bow. For example, if at both ends of the 16" reference piece, the fully drawn bow shows a gap of 1" on one limb and 3/4" off the other limb, then the stronger dip area requires correction provided that the gaps were equal upon the braced bow as it lay cradled upon the tillering tree. If in your judgment both dips are balanced but extend too far into the limb, this close visual inspection will aid you in finessing them further into the handle area.
A variation on this technique works mid-limb on the humpy-bumpies, or even upon limb tips where you want to slow movement back down again. I usually use a curved piece of stock that roughly conforms to the arc of the braced bow upon these areas because the gap widens too much between the limb and a straight edge referent, making it difficult to judge relationships accurately.
When clamping over trouble spots on the limb, be careful not to clamp over a valley so that the referent has two contact points. Clamp always upon a bump so you can watch the limb to both sides. When clamping upon limb tips, you will need to clamp both of them simultaneously so the bow remains balanced and does not tip out of its cradle.
Though it is true that the perfectly tillered bow reveals itself only for a glorious moment by the arc it traces at full draw, and that final judgment of tiller derives from the sight and feel of this tensioned arc, it is also true that you sometimes need to get to it by taking small steps. A reference stick can be a step in that direction.
"Carve a little wood, pull a few strings, and sometimes magic happens." Gepetto