What accounts for the strange expression on the convict's face as he looks at pip?

"As one of the soldiers, who carried a basket in lieu of a gun, went down on his knee to open it, my convict looked round him for the first time, and saw me. I had alighted from Joe's back on the brink of the ditch when we came up, and had not moved since. I looked at him eagerly when he looked at me, and slightly moved my hands and shook my head. I had been waiting for him to see me, that I might try to assure him of my innocence. It was not at all expressed to me that he even comprehended my intention, for he gave me a look that I did not understand, and it all passed in a moment. But if he had looked at me for an hour or for a day, I could not have remembered his face ever afterwards, as having been more attentive."

The convict knew that Pip had just assisted him, and that he wouldn't say anything further....... his "attentive" look almost made them accomplices. I think this is the moment Magwitch decides that Pip is honorable and well able to become the man he wished he could have been.