Sliding forward on the saddle is a natural thing to do - when saddles were riveted, riders would describe going hard as "on the (front) rivet". The expression has outlasted the rivets.
No one does this consciously - you'll see racers who are struggling (in solo breaks and the like) slide forward on their seat, push themselves back, and repeat ad nauseum.
I like a forward position. It helps put weight on the front of the bike (excellent for all but steep downhill corners), it allows me to raise my saddle/torso a bit, and I get much more short burst seated speed.
- easier to spin when forward
- easier to breathe when forward
- recruit upper body naturally and effectively
- maybe a bit more power due to effectively lower seat position?
- personally I have a LOT more power when forward, as evidenced by powermeters
- one step away from standing up
If you commit to a forward position (TT bike, triathlete) then you need to raise your seat a bit. This will allow you to get proper leg extension.
I find a forward position is pretty much unusable at lower rpms - that's when sliding back on the seat helps, like on long climbs. On short or very fast climbs I actually sit on the tip of my saddle and spin like mad, but since I rarely climb fast, I usually resort to sliding back on the seat and slogging my way up.
Since mass start racing has all sorts of efforts, a compromise position is good. Medium position, allowing you to slide forward or backwards 2 or so inches, with the resulting different leg extensions allowing you to recruit different muscles.
cdr