West Ham thought they had levelled at 2-2 and referee Andy Madley was initially happy when Maxwel Cornet put the ball in the net.
It was checked — as all goals are — by VAR, in this case by Jarred Gillett. He saw what he thought was a foul in the build-up where Jarrod Bowen caught Chelsea keeper Edouard Mendy before the ball broke to Cornet.
Gillett recommended a review; he went to the referee review area, took several looks and decided to disallow a goal.
In my opinion, there was a trailing leg and some contact, but had the original decision stood and the goal been awarded, Chelsea could have had no complaints.