Not sure how similar this is to the ETH implementation, but for example,
The #100 RANDAO + VDF round generates a random number that selects validator X1, X2 etc to make block #1234, #1235 etc, X1 signs the #1234 block including transactions A, B, and C.
However, during the #100 RANDAO + VDF round, one of the people contributing to the randomness pretends to be offline during the round but make up a number after learning the others’ VDF results to make a fake round pointing to his/her addresses to make #1234, #1235 etc blocks. He/she makes the blocks but only includes transactions D, E and F in it without transactions A, B and C and then continues to build on and send out this chain and the fake RANDAO + VDF data.