In the early days, someone at a trade show came to me and discussed adding a Tachistoscope functionality. I was intrigued and took his phone number. We had a long discussion on the phone as to exactly how it worked and why it was developed. Still to this day I have found much information on it, but I still receive e-mails once in a while from users who have been using the Tachistoscope and give the feature high marks.
The T-Scope allows you to granularly show the various sides for specific amounts of time, with selectable delays in between the sides of the cards. Supposedly it was developed during WWII for fighter pilots. They were shown silhouettes of various aircraft for very short periods of time (typically in the milliseconds) and made to decide which air craft it was and whether it was friend or foe. Seems like a pretty good thing to know if you are a fighter pilot.
I'm not sure what people use this for today, but I have always been curious. I also wonder about that early conversation and whether that user ever ended up using the T-Scope in Memorize-It. I know there are a lot of features I have added over the years that the specific user that asked for them never used, but overall the process had made Memorize-It a very robust flashcard system.