Doing it slow motion mentally is like watching a slow motion replay so while playing it our in your minds eye you're doing an action exactly as you would do it in reality. The action in the mind will take say 10 seconds to play out in your minds eye and when you go our and do it, it really only takes 2 seconds.
You could also grab some videos of people doing the tricks and visualise yourself in their place... play it in slow motion too. 
Alex
