Yes, for each token only, say, 10% of the weights are necessary, so you don't have to fetch the remaining 90% from memory, which makes inference much faster (if you're memory bound; if you're doing single batch inference then you're certainly memory bound).

As to how the selection works - each mixture-of-experts layer in the netwosk has essentially a small subnetwork called a "router" which looks at the input and calculates the scores for each expert; then the best scoring experts are picked and the inputs are only routed to them.