This is a translation of the lore of dashi (山車, large float-like vehicles used in festivals) from the book “Me de Miru Hachioji no Dashi Matsuri” (目で見る八王子の山車まつり), which comes from my parents-in-law’s hometown in Hachioji, Tokyo. This is but one of the resources used when writing an earlier 2-part article on the same topic. To read those articles, click on the links below.
Experiencing Japanese Festivals ~ Part 1
Experiencing Japanese Festivals ~ Part 2
The Japanese text from the book is presented along with my English translation. Some extra text for clarification is included to better understand.

Dashi is a “shimeyama”, which bears the meaning of being a medium occupied by a god. It is a structure known by a variety of titles such as hoko (spear), yama (mountain), shinboku (divine tree), and gohei (Shinto staff with plaited paper streamers). A shimeyama is but one of the devices used for transferring the shintai (divine object of worship) in festivals from the main shrine.
In the book “Takaoka Goshayama to Nihon no Hikiyama” (compiled by Ise Souji), it is written that the origins of today’s dashi have a deep connection to historical versions like the Aoba yama found in the “Kiki” (2 mythological books regarding Japan’s creation called “Kojiki” and “Nihon Shoki”), as well as the man-made shimeyama used in the grand ceremony within the Imperial palace during the year 897 (early Heian period). From the same text, we get an idea that it is called a mobile seat of the gods, for it is written that during the 1st year Kanji period (1087) in Kitano of Kyoto, the shimeyama was pulled from the Tenmangu (Tenman shrine) all the way to the Imperial palace. Also mentioned is, during the 1st year of Tennin (1108), many people flocked to bear witness as the shimeyama, a representation of prosperity, was pulled along the Nijo Suzaku main road in Kyoto. It is speculated that the hoko, the mountain that was pulled during the Gion festival in Kyoto, was the beginning stage for the shimeyama.

Man-made mountains like the hoko, which bears a long history being used in religious festivals during the Gion celebration in Kyoto, garnered much attention that years later, from the middle ages all the way to recent times in Edo period, word about it spread to each area throughout Japan.
They then started to appear in other areas as part of the celebration during religious festivals where a shintai was carried around from the main shrine. Over time, their shape and the ornaments they were adorned with began to change. Also, during the start of Edo period, many types of dashi possessing their unique traits were being built in each area throughout Japan. Examples of this are those made famous by being listed in dashi pictoral catalogues, such as the kabuki yatai (floats that look like a kabuki theatrical stage), karakuri dashi (floats that feature puppets), and dashi that feature dolls of well-known individuals from Edo period. Out of these are those few that possess impactful designs due to being modified versions of the older shimeyama, such as the kasaboko (beautifully adorned floats with umbrella-shaped tops) of Chichibu City in Saitama Prefecture, which has a shape designed as a device for transporting the object of a god has a shape, as well as the Tanakayama of Nagoya Prefecture, and the man-made ippon bashira (floats featuring a large single-column) of Edo (former name of Tokyo).
From this viewpoint, we get the definition of the phrase “shimeyama’s lasting impression”, which is visible through the ippon bashira dashi of Hachioji. What remains is the folklore that influences the dashi’s change in shape, as well as development of people’s way of life and the sense of establishing a culture of higher quality.

By the way, the original word for dashi comes from idea of a spear where the gods can descend down onto the spear tip, with a setup similar to an old-fashioned encased basket with protruding bamboo tied together called a “higeko”. This would then be called “dashi” (protruding outward), becoming the verbal root word for the mountain float
(From Orikuchi Shinobu’s “Higeko no Hanashi”)