Manacle

A Manacle is an ancient talisman created by the Spirit Oracle to help the Humans protect their land. The first Manacle was given to King Arachna the First at the Hill of Champions. The Manacle allows humans to bond with their very own Battle Spiders. Whenever the rider activates his armor, there is an upgrade in the spider's armor as well, hence the connection.

The one who wears the manacle becomes a battle ready warrior when they say "Arachna Power" or "Oracle's Light". When that happens the gem on the manacle glows and webs shoot out and turn the riders civilan clothes and gives them a shield in weapon, it also makes their spider's armor stronger. They can then change back by saying "Rider Out" to go back to their civilian attire. They can summon their spider partners from inside their manacle by calling "Spider Out" and the spider's name. They can also dismiss their spiders into Manacle Space by calling "Spider In".

Manacle Space
A Manacle can convert any spider to a form of energy, pull it into its center, and close. When this happens the spider will enter into a pocket dimension called manacle space.

The environment inside manacle space is designed to be attractive to spiders also; according to Shadow, he felt instinctively at ease within manacle space.

Spiders appear to be conscious while inside manacle space. Several spiders have shown the ability to leave manacle space at will, most notably among them being Shadow.

In Episode 52 the Manacle came off their wearers because of all 4 Oracle Keys. They made into a formation with the keys at the middle. Soon the powers of all 8 Spider Riders and the 4 keys combine so those powers went to Hunter and Shadow who transformed into a new form. After the final battle the Spider Riders were shown wearing them again.

Transformations
Transformations occur when a Spider Rider calls upon the Oracle. The two phrases used to do that are "Arachna Power!" which is said by the majority of the riders or "Oracle's Light" which has only been spoken by Aqune. When a rider wishes to go back to their civilian attire they say "Rider out!" which is only said in one episode.