snubbing is a flag (computed, but that you can change manually), that indicates that a peer tells you he will send you data, but doesn't. When a peer is snubbed, you don't trust him anymore, until he gets un-snubbed. So you'll only queue him 1 request, and moreover you won't mark that block as being downloaded (so you'll request it from an un-snubbed peer too). A peer gets snubbed by taking more than 1 minute to send you a block (16k , so that's less than 300B/s), he gets un-snubbed if he sends you a block in less than 45 seconds.