CathInfo.com

Here you can find all you ever wanted to know (and more) about why DD is, in some circles, called One-hand Dan.

http://pistrinaliturgica.blogspot.com/

During the 1976 ordination to the priesthood by Archbishop Lefebvre, a class which included both Daniel Dolan and Richard Williamson, Archbishop Lefebvre evidently only laid his right hand on the heads of the ordinands, contrary to the rubric of the Pontificale Romanum, which indicates that the bishop lay both his hands on the ordinand's head, perhaps due to his having the day before ordained deacons (where the rite calls for one hand).  In any case, the argument was made that this is invalid.  So many of the SSPV consider Fr. Dolan not to be a valid priest and therefore not a valid bishop either.  I know that the SSPV conditionally confirms people who had been confirmed by Bishop Williamson.

Now, the Eastern Rite has always performed ordination to the priesthood with one hand, the Roman Rite also did for a couple centuries, but then switched to two hands, and I believe Father Cekada found some ruling on the matter from a Vatican office which indicates that this would in fact be valid.