N-heterocyclic carbenes are similar to cyanide in reactivity. Like cyanide, they have an unusual chemical ambivalence, which allows them to trigger umpolung in reactions where they are involved. The carbene has six electrons - two each in the carbon-nitrogen single bonds, two in its sp-hybridized orbital, and an empty p-orbital. The sp lone pair acts as an electron donor, whereas the empty p-orbital is capable as acting as an electron acceptor.
In this example, the β-carbon of the α,β-unsaturated ester 1 formally acts as a nucleophile, whereas normally it would be expected to be a Michael acceptor.
This carbene reacts with the α,β-unsaturated ester 1 at the β-position forming the intermediate enolate 2. Through tautomerization 2b can displace the terminal bromine atom to 3. An elimination reaction regenerates the carbene and releases the product 4.
For comparison: in the Baylis-Hillman reaction the same electrophilic β-carbon atom is attacked by a reagent but resulting in the activation of the α-position of the enone as the nucleophile.