Theres 2 reasons firstly you need to get the back hat toss out, secondly the initial knock down is neasesary for this combo as its spacing specific. You can't use B321 > Back hat toss or when you do the teleport 3 > 44 > run > B321 > hat call back you will have traveled passed the hat and drop the combo. try and do it yourself with any other knock down that is cancelable into the back hat toss and you'll see what I mean.
This is a two part combo that changes slightly dependant on what string you use to cancel the back hat toss into.
Long story short use a string of your choice if your opponant is blocking use EX hat toss as it leaves you plus 30 on block and hits twice this gives you time to use an over head string or use F2,3.
F2,3 won't lead into this combo but you see thee purpose behind the initial knock down now if you get it.
Go into the combo I put in the video.
IF you land something else eg B321 do this .
B321> back hat toss > teleport 3 > 44 > hat callback > F21 > spin > jump kick > dive kick >f1 > forward hat toss > low block.
Then use the hat call back to start a combo again.
Ps the f22 was a typo thanks for correcting it
also did you watch the video because if you did you'll see why I do what I do. Here I'll post it again.
http://xboxclips.com/TM SOUL CAPTAIN/98773fec-cd52-447e-a833-f3910ce746f7
Hope that answers your question.