this guide talks about how to perform speedghosts and polswid tech, while mentioning a few of the major spots its used. It can be used in more spots than what is mentioned in this video so I highly suggest watching time trial WRs on CTRanking to see if there are any usages that have been found since the release of this video.

SOME UPDATES: re: turbo pad's effects on polswid (mentioned at 9:18) - unique turbo pads (all blue fire pads, and every turbo pad in the game that gives fire that is not the standard sacred fire speed) will actually give you a large boost of speed due to the game thinking you are at base speed with no fire and giving you the +(x speed value) bonus of the pad, and polswid's momentum preservation will hold this extra speed given (this can be most easily observed in recent koala carnival runs). Standard (sacred fire) pads will also give you a burst of speed but that speed will go away when the fire dies out unless you perform a wall-scrape before the fire naturally burns out. (Go watch recent top level roo's tubes runs to see this in action)

re: hangtime boosts (mentioned at 9:35) - you can get a hangtime boost in offroad sections and maintain that momentum UNLESS you exit the offroad and enter the road in the air. (so for an example, jumping at the end ramp in roo's tubes, getting a hangtime boost by landing in the offroad, and then hopping onto the road will kill your speed, BUT if you just drive on the ground while transitioning back onto the road, your speed will be maintained)

re: speed dying out mid-air (mentioned at like 11:15 and i rambled about it for a few minutes) - if you're in the air for a long time without it being a downhill, your speed will go away. if you hit a boost pad and that fire goes away midair, your speed will go away. From my understanding what's happening here is that the polswid tech isn't really applied in the air, so losing fire mid-air while not on a downhill section will make the game try to set you to the correct speed. Similarly if you float over an uphill zone long enough it will attempt to reset your speed as well. You also seem to lose speed if you land on or just before a turbo pad. I have no idea why this happens.