How can characters who can't sense ki able to see the action?

It is explained to Gohan during the Yamcha vs Saibamen fight that you can't see fighters after they go a certain speed and you HAVE to use ki sensing to keep track of them.

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But how can Vegeata and Nappa follow the fight then? If they had their scouter on I could beleave the devices could act as a replacement to ki sense but they did not have them on, so again how???

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Same goes for Freeza and his forces.