You flick through the slides of your presentation on the way to the venue in the taxi. The slides look great, the story is perfectly clear.
Not anymore when you are on stage.
A live rehearsal is the only way to go. And not only to practice stance and eye contact (with the mirror in front of you).
You need to switch your brain from parallel to sequential processing. An image says more than a thousand words. If you look at your own slide it all fits together perfectly. That image, the diagram, those 2 words, the pressure of the 2 opposing arrows. For you (the slide designer), it triggers a complex set of thoughts in your brain. 
The audience does not have any of this. You need to translate that complex mental picture into a sequence of thoughts and sentences that allow your audience to get that same insight.
The only way to do that is to "switch off" your parallel visual thinking and start listening to your own sequential stream of words.

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