Another Book % question

[doubleback]

Analogy ...

[seanmould14]

Hi all, i get that when the book is slack a horse can carry on its move without another horse moving, and when its tight, if one steams, another has to drift to stop the book going below 100% (please correct me if im wrong).

What i don't understand is what causes the book to be slack or tight? Is it anything to do with the liquidity? Or just something simple like only one horse has moved? Just can't get my head around it!

Cheers
Darren.