Alexender Phase Detector in CDR??

Hello,
Please, can anyone explain how does the Bang Bang Alexender PD work?
How does it get lock?
i need brief explaination, as i tried to see it in Razavi Optical communications book, but it's bit difficult,

Thanks in advance,
Ahmad,

it is a bang bang , i.e. it oscillates around the steady state position , it gives some early and some late , till it reach a steady state where it osc. bet. early and late.
the idea is that F1,F3 and F2 samples the data by the c/k at +ve , -ve and +ve c/k edges respectively"F4 is just to sync. the results at the xor i/p".
when sample1 == sample2 != sample3 then the clock early
when sample2 == sample3 != sample1 then the clock late
when sample1 == sample2 == sample3 then no transition and hence x==y >>Icp=0

p.s. there is a condition called metastability but we didnt take it , so just consider it bang bang.
the metastability happens when s2 is sampled at the data edge >> low o/p >> both X,Y>>low and there is no pumping of current>> holds its position

Thank you Safwat for reply,
but i think that UP and Down in my figure above should be reversed as follows:

yep , i usually work it with x,y but i understoond it now,thnx