Personally I prefer the method of creating a folder called "Screen" etc (instructions below) I can't find the original post, but fortunately had them copied down.
Method 1.
How To Capture a TT Screen.
1. Create a directory in the root of your TT and name it "Screen" (without the quotes)
2. In the new Screen folder create an empty .txt file and call it "Capture" (once you save it remove the .txt extension!)
3. Re-start the TT and test.
4. Capture by tapping the upper-left corner (notice the camera sound)
5. The screen-captures are stored in the new Screen folder.
6. To temporarily disable it just slightly re-name the Screen folder (maybe to "1 Screen") without the quotes.
Lets face it, you will probably be using it when hooked up to the computer so it is actually the easiest method to use.
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Method 2.
The link to the "Screenshot On/Off" method "which I will say I did not like" because it keeps making the TomTom re-boot each time a action is done.
http://www.navitotal.com/forums/showthread.php?6228-Screenshot-tap-button-on-9.051-SE-navcore&highlight=Screenshot+ON-OFF.