If you keep your task management system absolutely up to date, i.e., it stores everything you plan to do eventually, then you really can trust it and relax in the knowledge that you don't need to track TODOs in your head. It becomes an extension of your brain, leaving your primary brain free to relax and focus on whatever you're currently doing, even if -- especially if -- you're not doing anything in the present moment. But this doesn't happen if the system is leaky, i.e. you slip up and start tracking some TODOs in your head.