Some readers leave the words on the page, instead of making the extra effort to visualize the words in their heads. This will affect fluency, comprehension and the overall ability to read. Flashing words, and later phrases, helps readers over this hurdle. Always use words or phrases the reader is able to read but reads slowly, gets the letters mixed up, or reads very robotically.
When using flashcards, turn one over and place it on the top of the pile to cover the first word. The reader will be looking at a blank card. Raise the cover card for just a second, and then return it. This way the reader has only a second to look at the word. (Make sure to raise the cover card and leave the word to be read still.) The reader can now take time to think about the word and figure out what it says. If the reader struggles, ask what letters the reader saw. If the reader can spell the word correctly, use the spelling to help read the word, or if you need to, write down the letters. If the reader can not spell the word correctly, flash the word again. At first the word may need to be flashed many times before the reader can put the image of the letters in his or her mind and read the word from there. After a while, this should get easier and easier.
Good readers read unknown words in chunks, and they read sentences in chunks. As you are reading this, you are actually looking at and reading a few words at a time. Once the reader is comfortable reading words using the flashing technique, it is great brain training to read common phrases using this technique.
If words or phrases are in lists on a piece of paper, the same technique can be used. Just cover all words not being read at the time and with a separate piece cover the word or phrase to be read. Remove and replace the cover on the word to be read as described above. Often readers will not need the words and phrases already read to be hidden from view, but hiding anything from view that has not been read helps the reader know where to focus to see the word or phrase being worked on.