Pendlay rows as far as I understand them are about taking the hips out of the movement and eliminating excessive jerking of the weight.
I use the same grip as my bench press and row low to the stomach. Hip position doesn't change. I like 25lbs plate for better lat stretch because you are supposed to let the upper back round on the deweight. Pull hard with arms and arch your back at the same time. Try to feel the contraction in your lower lats/mid-back.
Fully deweight after each rep.
Supposedly Glenn did emg work and found that this style of row activated the lats the most.
I have to be honest and say that it has turned into my favorite row after getting used to the groove of the movement.
The main difference between this and how I used to do rows is keeping the hips stationary and really focusing on arching the back as you pull up to maximize the contraction at the top. Back still shouldn't go much above parallel. The smaller plates helps facilitate this too I think.
The pull and the arch of the back should be one move. Pulling and arching back hard at the same time.
If I row to mid to low chest and arch hard I feel it in my traps and rhomboids alot. I prefer to go low around mid stomach and feel the contraction in the lower lats, but both are good.
Takes abit of practice getting used to pulling and arching at the same time without using the legs or hips, but I think it feels really strong and has solid isolation for a compound movement.
That's my novice take on it.
Oh and I use a thumbless grip as well. I prefer thumbless and BB rows, but regular on DB rows. Think it has to do with wrist position with the wider grip. That and I remember an old black and white photo of arnold doing them thumbless. That probably contributed somwhere.