this seems to be getting a little technical now, so here's my take on it
All defenders set to blitz have a pressure radius, and will project "pressure" on the QB, when the QB is in that radius.
This is why 30 Speed as a bare minimum is important for QBs. Speed controls the drop back. If the QB cannot drop back fast enough, he'll never escape the pressure radius.
The effect of pressure is specifically to decrease pass quality.
The closer defenders get, the more pressure they apply, and the more pass quality goes down.
What confidence does is reduce the pass quality hit applied by pressure.
If the pressure effect is bad enough, and the confidence effect is not high enough the result will be a bad pass.
So, in theory, if you had a QB with 50 Speed, with an amazing o-line, and he only threw short routes, then he wouldn't need very much confidence at all because defenders wouldn't get a chance to apply as much pressure.
A play like this (from the All-time Top Plays)
http://glb.warriorgeneral.com/game/replay.pl?game_id=2193701&pbp_id=5488262requires high confidence (also Pocket Presence SA). The DT was right on top of the QB, and the LE and RE were closing in.