Watching an LGR video and he mentioned it. I always assumed they were just some kind of tension relief or a splicing point for repairs or something, literally never knew they had a more technical purpose.
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They act as cheap low pass filters.
Alternating electric current induces magnetic field that opposes the change in electric current. Faster change (higher frequency) induces stronger field.
Adding loop of metal around the wire makes the effect stronger.
So the metal block will cause high frequencies to be attenuated in the wire. The block is made such that it mostly affects only frequencies that are higher than the frequencies used for data transfer.
The device you’re talking about is a ferrite bead. It acts as a kind of device called a “choke”, which serves to eliminate unwanted high-frequency signals that can cause undesired operation.
Basically, any length of wire, including a data cable, acts as an antenna, picking up stray electromagnetic interference (EMI). This interference can cause signals to appear on the cable that aren’t supposed to be there, which results in undesired operation- lost signals and data, intermittent operation, and so forth.
A ferrite bead choke acts as a low-pass filter that “chokes” the high-frequency EMI that is induced on the cable from external sources. This helps to reduce or eliminate unwanted interference from external EMI.
I’ll try to make this a true eli5 answer.
When information moves through the wire, it moves like a stream of water, in a “current”.
The current can pick up stray bits of trash on the river bank, dirtying up the signal.
The cylinder acts like a concrete culvert, designed to stop trash from falling in the river; and a net that catches the trash as it flows through.
As the electrons’ fields move past the ferrite, the iron repels the magnetic force – like a strong wind will slow you down walking in to it.
It works a treat on sheilded cables, but ideally you want to put many loops through a ferrite.
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