To make a long story short: the advice here is to, first, remember and visualize the problem and than, second, to solve it only in your mind without any visual reference.
IYou should begin with very (very) easy positions and go on to more complex after you were able to solve the problem 100% without looking at a board&stones or a page with a printed problem or whatever...
You can do this in a lot of moments during your every day life where usually study tsumego is not possible... for example while washing the dishes.
Back when i first read about this technique my thought was "ok, good idea but i will never be able to do this, i will not even try this"
... but yesterday i somehow managed to, no — not to solve yet, but, as a first step, to memorize a position clearly in my mind.
That was a very exciting feeling... and i will try to continue today solving the problem.
It seems like a great method to improve reading, and it is fun! You should try yourself.
I think there may be 4 phases with this:
1. trying to visualize the position (remember start with very small and simple problems!)
2. beeing to able to visualize clearly
3. trying to solve
4. actually beeing able to solve the problem and doing it again with a bit more complex problem.
I'm in phase 3 :)
Good luck with trying this out and don't think you can't do it, you can! ;-)