To clarify: this is the part described as Buffer (Tubestuff).

Its proper function is it will accept items as though it was a chest, and it will push items into a wooden pipe without needing a redstone engine, and it will issue a redstone signal to operate a filter.

There's a quirk with the wooden pipe: It tries to get the pipe to point outwards but if you place the pipe first and the buffer second you can get it facing the other way. It ignores the direction, but if powered the pipe still operates leading to "ping pong".

The redstone function is more interesting. Its a weak signal that doesn't propagate to dust or vanilla repeaters, but it will trigger redpower logic, especially the state cell. It will also operate transposers and filters placed alongside it even if they are pointing in different directions, so if you have buildcraft bits spare it may make a cheap substitute for a timer.

It only pulses if there is an item in it and the more spaces are filled the faster it pulses meaning IT IS POSSIBLE TO DETERMINE HOW FULL IT IS BY THE CLOCK RATE.

How you do this is up to you but a starting point might be to put the buffer after a machine, and have a timer placed next to the buffer, set to around 2 or 3 seconds. Link that timer to the upstream transposer/filter feeding the machine and when the buffer gets about half full the timer will be reset before it pulses so the supply of materials to the machine will stop.

Might be time to un-sticky this. Tubestuff still continues in some form but the inventory-sensing trick is now better implemented with a hopper and a redstone comparator.