I have always been very confused about whether the tip line on the receipt in the US works with my British cards given that I enter a PIN into a terminal that doesn't show that tip amount.

As of last year I'm pretty sure the tip is deducted from my card, but I don't think that has always been the case. I understand it works based on PIN-authenticated pre-authorisation for a higher amount and they later take your tip+bill from that pre-authorisation.

It doesn't seem very secure but the US always seems behind on card security.

When I first started travelling to the US for work restaurant staff were always extremely confused about why my card needed a PIN. They often tried again and again or said my card wouldn't go through, then worked out that it needed a PIN. Lots of places then had no way to hand you the terminal to enter it, like they would have to push aside mountains of junk to get the terminal out, or invite me round to the other side of the bar because it's literally screwed down.