How does this part work? ( refriderator compressor start relay )

Yesterday, my GE refridgerator stopped cooling. The compressor would not turn on. I discovered that the “start relay” was the problem. ( See pic#1.)

I took the bad “start relay” apart and found it contained a “ceramic”? disk on the inside, about the size of a US nickel, in thickness and size. Mine was broken into several pieces so would not work. ( See pic #2 ).

Once I replaced this “start relay”, all is working now and has been for 24+ hrs. Here is a pict of a NEW “start relay”, that I opened up to see the ceramic disk… ( pict#3 )

HERE is the question… What exactly is this ceramic disk? How does this function? It is like no “relay” I have ever seen. What is the electrical theory behind this? Thank you.

Watch all the way to the end. Very informative.