Detection is recompute and compare. Changing one message bit changes the CRC produced by the same generator.
highlighted = computed this step
Why compare instead of verifying bad data
Verify mode rejects a bad codeword, so this lesson recomputes the CRC for the changed message. The mismatch with the received CRC is the detection signal.
received CRC=100
One message bit changes
The original message 11010011101100 changes to 11010011101101. The generator stays 1011.
11010011101100→11010011101101
Recomputed CRC differs
The recomputed CRC is 111, not the received 100. The receiver detects the error by comparison.
111=100
Summary
CRC catches every single-bit error and every burst error up to generator degree 3, plus many others; a corruption that is a multiple of the generator can slip through. It never false-alarms on a clean codeword. Polynomial arithmetic over the exact bits; timing/throughput is not modeled here.