The answer is here http://www.signalbox.org/block/absolute03.htm
It's a UK 'belt and braces - and an extra bit of string to be on the safe side' practice to ensure trains are kept well apart. What it would mean on a model, essentially, is that if you're shunting on the main line you can't get a train drawing up to the home signal (or if you're modelling two stations with boxes, a train can't leave one box while there's shunting on the main line at the next box).
Of course, there are exceptions - a list as long as your arm.
If that would lead to frequent delays, the powers that be may have authorised Regulation 5 (as was) - the 'section clear, station or junction blocked' described here http://www.signalbox.org/block/absolute05.htm
so a train can be accepted while shunting goes on. Of course, even that won't (hopefully) lead to a train drawing up to the home signal while the shunt is in progress; there's likely to be a visit from the DI if a signalman does that sort of thing, unless the arriving train is a freight that will itself be shunted around on arrival. The purpose of the regulation is simply to keep things moving more freely, allowing shunting to continue until the last minute. (And of course most Reg 5 authorisations wouldn't permit a passenger train to be accepted that way in any case, so you'd virtually never get an express arriving with shunting in progress. But authorisations could be whatever they needed to be depending on circumstances; if you decide what your situation is and ask on here, I'm sure someone will have an example of it having been done somewhere.)
Of course if you have an outer home 1/4 mile in rear of the inner home and you shunt inside
the inner home only, you don't need to block back at all. Again, the bobby would expect a visit from the DI if shunting was kept going until a 4-beat got a check.
Then there's all the possibilities with goods lines and permissive working. But again, on a model that's not likely to make a lot of difference unless you're implementing full block working for the operator(s).
Which may not be a great deal of help, I'm afraid. If you think it's going to be a feature of your layout, why not put up a diagram somewhere and se what folk think about how it would actually have been done?