He was with the rest of the team but at a roundabout he was the only one to take his way round so was instantly isolated when he punctured at that point for a minute or so.
http://www.cyclingnews.com/news/porte-team-sky-are-fired-up-to-gain-back-time-at-giro-ditalia
The incident happened with just over four kilometres remaining of the stage, meaning that Porte was outside the safety of the three-kilometre rule that prevents riders from losing time if they have a mechanical or are involved in a crash. Porte’s teammates were on the opposite side of the road when it happened. When they finally made it over to help their stricken leader Clarke had already stepped in, unwittingly costing the Team Sky rider more time.
“By the time the guys had got back to me Simon had already stopped and offered me his wheel. It was a spur of the minute thing,” he explained. “Alberto summed it up last night - all you are thinking about when something like that happens is 'how can I make sure I lose the least time possible'. I didn't even give it a thought that it might be breaking the rules. Everything was happening so quickly and I was just acting on adrenaline.”
