It is just another method of harvesting yeast.
Gt from Grumpy's originally put me on to this method
Ideally it is a method for pitching straight after drawing it out, but you can store it in the fridge until you want to pitch it.
You are drawing it out at its most active, rather than waitng for it to die down
You ideally want to be drawing it out at around the halfway point of your SG & FG... so about the 1020 mark
You need to draw out the same amount you pitched, so if you draw out 2 ltrs, pour the whole lot into the next ferment, then draw out the same amount, repeat
The good thing about it is if you do store it in the fridge, then when it comes to pitching, leave it to warm up for a few hours to the same temp as the wort and you start getting a krausen again and pitch it straight in
A very economical way for yeast harvesting if you are brewing similar styles