Other threads explain replenishment and its benefits more in detail, but just briefly for anyone else reading who might be interested:
To replenish XTOL, you mix up your 5L stock and then divide it into two (or more) bottles that you designate as developer and replenisher. Every time you develop film, you pour it right back into your developer bottle, except for 70 mL (if I recall correctly) per roll developed, which you discard down the drain. You then fill up that space with fresh stock from the replenisher bottle.
Terry has it right, although I put the correct amount of replenisher (70 ml per roll in the case of XTOL) in first, and then just pour back the recently used developer until my "working developer" bottle is filled to the brim.
In any week that I don't develop film, I just do one roll's worth of replenishment instead - it keeps the developer fresh, and you don't have to test. I would recommend some testing though after 6 months of use.
There is one further advantage. Your developer is always mixed up and ready to go. I always develop at whatever the ambient temperature of the developer is - adjusting the time as required. It is the same for fixer and HCA. The only temperature I have to adjust is the stop and the wash, and I generally can just let the stop come naturally to room temperature.
