No you just tap out again and in most cases you will be charged correctly.
'Most' - as opposed to 'always' when using a paper travelcard...
Ok, so this is merely the start of what I suspect is a long list of 'things I don't want to have to know', but looking that up on TfL's pages says
https://tfl.gov.uk/fares/how-to-pay...-and-oyster/pay-as-you-go/touching-in-and-out
If you touch contactless (card or device) or an Oyster card on a yellow card reader when you enter a station, and then touch out to exit it within a short period of time, you'll be charged a same station exit:
- Between 0 - 2 minutes: a maximum fare. If you re-enter the same or a different station within 45 minutes, you'll be refunded. This doesn't apply if you take a bus or tram before re-entering a station
- Between 2 - 30 minutes: minimum pay as you go fare from that station
- More than 30 minutes: we'll assume two separate journeys have been made and both will be incomplete. You'll be charged two maximum fares
That's already significantly more complex than using a Travelcard. So if I go onto the platform and see that the next train is 20 minutes away, I have to remember to wait two minutes before trying to leave again, else get a massive penalty? (Or possibly not, depending on what I do in the following 45 minutes). And I don't see any 'no charge' option, which is of course the case with a Travelcard.