Progressive anagrams are best explained by example. Consider the haiku:
What frisky anxious
monster doesn't consider
The final word of the haiku is the keyword. Spelling this word backwards will allow you to determine a spectator's mentally-selected word. For example, you ask whether the selected word contains an "s." If the answer is "no," then the selected word is "what." If the answer is "yes," ask whether the selected word contains an "n." If the answer is "no," the word is "frisky." If the answer is "yes," continue spelling your keyword backwards until you get a "no" response. There will only be one "no" response and the letter it falls on reveals the identity of the chosen word. A "no" on "a," the 6th letter you spell, tells you that the selected word is the 6th one.
This program will help you build your own progressive anagram. Enter a keyword containing 1 to 9 different letters. You will get a (very) long list of words from which to choose your progressive anagram. For example, if you give the keyword "maidens," the word "what" will appear among the choices for word 1, the word "frisky" will appear among the choices for word 2, and so on. You still have a lot of work to choose words from among the many possibilities. I wish you luck.
Letters should not repeat in your keyword. For example, "sugars" would have you asking about "s" twice, which makes no sense. You could, however, use the keyword, "ugars." "Sugars" could appear in your list of words, but you would stop spelling at "u." Similarly, the word "hellish" would use the keyword, "elish."
Some words do not allow for a complete solution. "Explain," for example, leaves no choices for the seventh word, but "xplaine" does. Thus, "explain" could be in the word list, but you would remember "xplaine" as the keyword. You could also use fewer letters, for example, "xplain" or "eplain" as your keyword.Enter a keyword for your progressive anagram.
Note: Alain Nu gave me the idea for using the final word as a mnemonic for the letter-guess order. He credits Leo Boudreau's 1989 Skulduggery with the idea of hiding a progressive anagram within a sentence in order to use the last word to tip off the remaining words when spelled backwards. My contribution has been to automate the selection of the anagram sequence. I also came up with the idea of writing anagrams in haiku.