To be honest not much is known at all! There have been several news reports but they all contain the same basic information.
It looks like the actor in question got his leg stuck in part of the automation, most probably a lift of some sort (although that's just speculation), and cried out/ was screaming. At first audiences reported they thought it was part of the show but then, and this I find the most chilling part, he started screaming "my leg, my leg" and the audience realized was wrong. From that point forward there was a standard show stop (blamed on 'technical reasons') and the audience were sent home.
We're unlikely to get any more detailed/ real information because everyone on the show (like most west-end shows) will have singed a confidentiality agreement within their contract so they are not at liberty to explain. As for the adjustments before it reopens again today, I would imagine they would be more adjustments to blocking, e-stop systems/ procedures etc. rather than physical change to the automation systems and design but again that's just speculation.
Sam x
PS - It had been reported (can't remember where) that the actor in question didn't even break his leg so luckily he should make a speedy recovery. x