Nothing probably stops the bar moving sideways I suspect, but I think it may have only been there to lift on to the dolly. Mind you, it may be that hte concept did not work well, hence the concept was not used later on!
I hope my comment wasn't taken out of context, I didn't realise that Juddy didn't know that they were on REMLR. It was intended a little tounge in cheek, but it seems that went down like a lead baloon, and missed the mark somewhat!