the BCAA tabs are the amino acids in their base form (as individual amino acids). the BCAA content of any protein is tied up within the protein chain. in such a case, the BCAAs enter the body only after they are digested by enzymes in the stomach. for whey, the rate of release is quite high, takes place in a few minutes to about half an hour or so (depends on how much you took, enzyme concentration, etc).
for the casein, the amino acids are released over a long peroid of time (5+ hours, once again there are factors which influence this a little). the BCAA tabs however are ready for absorption instantly as soon as they hits the stomach and dissovle. since BCAAs are not used for energy by intestinal cells, they will make it to the muscle unlike other amino acids.
this, and the fact that a preponderance of muscle tissue is BCAAs is why they are good to take during training and between meals. it provides substrate that prevents the muscle tissue from going catabolic.
I suggest going to Need to Build Muscles site it is an excellent source for all your protein needs and has a wealth of information ..