In the early twentieth century, Annie Sullivan (Helen Keller's teacher) and Grace Fernald discovered the benefit of using movement to reinforce learners' understanding and use of letters for communication. Writing "d" as an extension of "a", for example, made it less easily confused with "b" than traditional printing, which starts both letters at the top, and leaves a decision to be made at the bottom of the first stroke - does the ball go to the left, or the right? With my current fourteen year old, this was not enough. She did not make errors in reading, but would confuse the letters when writing, even after practice.
The solution lay further back, in the production of the sounds "b" and "d" in speech. If we say "dad", we can feel our tongue touch our top palate, just above the teeth, and our lips don't touch. When we say "boy", we press our lips together to release the consonant (keeping our tongue low, though this is incidental to the teaching point). So, saying the word we want to spell gives us a direct and reliable indication of which letter to use. Lips touch - b. Tongue touches roof of the mouth - d. So far, it is working.