Manual spool braking is simply the application of thumb tension on the spool; the more experience you have at using your thumb to control spool speed, the less you need to rely on mechanical or magnetic braking features. Using your thumb to manually brake the reel is necessary, to greater or lesser degree, with every baitcasting reel, and no matter whether it has magnetic brakes (which some do), mechanical brakes (which all do), or both.
If you want a top notch baitcasting reel that will help you bring in that trophy redfish or striped bass then I would recommend you trying out the Abu Garcia Revo Toro 50 reel. This low profile reel with the large round side design is perfect for the big saltwater and freshwater fish.
If you’re completely new to baitcasting reel use, a magnetic spool brake is a good aid for quickly being able to cast short to moderate distances, especially with fairly large lures, with less time spent picking out backlashes. Since magnetic controls are accessed on the external sideplate, you can change tension at will. You’ll still have to educate your casting thumb to apply moderate tension, however, especially for attaining distance.
The mechanical system, using an adjustable centrifugal brake, is also a good means of controlling spool speed. Experienced baitcasting reel users only need a properly adjusted centrifugal brake, and a judicious but experienced use of thumb pressure, to cast flawlessly.