I read an expression in a sentence:

"Two hours into the climb, the wind kicked up so fiercely that it was all he could do to hunch down and cling to the rock, praying he would not be blown off the mountain."

What was all he could do to hunch down and cling to the rock? Praying? I don't understand the properties of the mysterious IT.

"It was all he could do to" is a very awkward expression. I wonder if it's even grammatically correct.

I can understand it better if it's arranged differently though such as:

All he could do was hunch down.
All he could do was to hunch down.
To hunch down was all he could do.

I just can't wrap myself around what "it" is trying to refer to.

I see your point. However, if the sentence before read "He had to take off his crampons." (or something similar), then it nearly works.

Although I'm with you, I do not think it's grammatically wrong, because there are contexts in which it works.