The psychologist Sophie Scott writes on The Conversation:
You can hear in their voices a slight embarrassment at having to discuss exactly what Botham’s inner thigh is doing, at the top of the clip. I can empathise. In this context, Agnew makes a little joke about “legovers” – but, very quickly, he and Johnston are not laughing because this is a very funny joke, they are laughing simply because they are both there, and they are laughing.
Provine also showed that laughter is highly contagious – one can often catch a laugh from someone else simply because they are laughing, not because of why they are laughing. And laughter primes laughter – this is why comedy shows use warm-up acts, as people will laugh more if they are already laughing.
In the legover clip they are pretty soon only laughing because they are both laughing – and they just keep on setting each other off (which is why Johnston pleads: “Aggers, do stop it!”). This behavioural contagion rather than the hilarity of the legover comment) is probably why this clip still makes me laugh, and I have been listening to it, analysing it and playing it to people for over ten years.