He said this the day before Mark Renshaw was ejected from the race for head butting Julian Dean. Both were trying to set up the run to the finish line for their teams' respective top sprinters, Mark Cavendish and Tyler Farrar. They and the other lead-out men had cranked up the speed of the peloton to over 40 miles per hour (70 kph). At this speed the tiniest mistake can cause a massive crash. Renshaw accelerated to pass Dean on his left and Dean put his elbow in front of Renshaw's to counteract the move. In most sprints Renshaw would then have countered by leaning into Dean with his shoulder. In this one, Renshaw believed he couldn't use his shoulder without bringing them both down so he bumped Dean's helmet with his own. He did this three times. This image, taken from video, shows one of these head butts.
Here's a video of the event.
Jon Brand reports that "As Renshaw hit Dean, one fan shouted, 'Zizou, Zizou,' the nickname of former French soccer star Zinedine Zidane. Zidane was famously ejected for head-butting an Italian defender during the 2006 World Cup final in Germany." (As it happens, I did a blog post or two on Zidane back then.)
As in the Zizou incident, no one was injured by Renshaw's unusual tactic.
Interviewed afterward, Dean said he was flabbergasted to get bonked on the helmet. He supported the officials' decision to send Renshaw packing. Renshaw said when Dean hooked his elbow over his own he had no choice but to use his head in order to keep his position. He said, "the object was to hold my line and stay upright."
Most of the articles I've seen say that Renshaw did wrong. They say shoulder bumping and elbowing are permitted in sprint lead outs, but not butting. It's also generally seen as ok to push with the head, but, as one coach says, a sprinter will get in trouble for using his helmet like a hammer. Others say head butts are a legitimate tactic for maintaining position. A former racer is quoted as saying "I’ve been mixed up in bunch sprints where I have been the lead out man or I have been having to sprint myself and people have head butted me. I've done it back. I've even taken my hand off to fend people off me."
Here's a smattering of news accounts.
Mark Cavendish's team-mate Mark Renshaw disqualified
Head butting is part of the cycling game
Tour de France Stage 11 turns into 'gladiator arena'
Head Butts Mar Stage Finish at Tour de France
Dean says ejection a fair punishment for head-butting rival
This photo, from Joost's website, shows him on the podium after he won the time trial in the Tour of Austria last week.