Unfortunately, they can't downshift to slow down because the transmission gears and shift drum are modified for drag racing. This modification is called an "Override Transmission" where the shift drum and the dogs on the gears are modified. This allows them to shift under full throttle without using the clutch. If they were to try and downshift it would bend the forks and destroy the transmission. If they were to abort a run they have to pull the clutch in when they let off of the throttle. If they don't, it would force that gear out and bend the shift fork. They use that same concept in Pro Stock cars also. Here's a picture of a motorcycle Override transmission. notice how the dogs have ramps instead of being square?
Here's a picture of Matt Hartford changing gear sets in a Liberty transmission. Notice how the dogs are ramped also? Same concept but in a Pro Stock transmission. Thats why when they run you never hear them lift off the throttle when shifting. Hope this helps.
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Here's a link to the video.
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