How do UK based UCITS funds trade when US market is closed?

I am wondering about the mechanics of UK based US-stock trackers which trade during UK hours but track US markets. I assume there are no timing opportunities here as the market in the UK trades going into the US open – but how is this achieved mechanically? e.g. if the US market looks like it is going to gap up or down, how is there no lag in the price for the UK security?