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Essentially, it's supposed to help you during the weak stroke when you pedal. Two sides of the same coin.
Whether it actually works on not, seems to depend on each person and the billion and one forum/video/blog/tweet/article you can be arsed to read.I have, and wish I didn't. @Vbulman summaries my thought on them now quite succinctly.
Chak
salad
Wikipedia says
*Biopace chainrings have a reduced chainring diameter coinciding with the cranks being horizontal. This is supposed to smooth the pedaling action, allowing the rider's feet to carry more momentum through the power stroke, and having it smoothly removed at the bottom of the stroke rather than encouraging riders to push bigger gears and risk knee damage due to higher knee joint loadings.[3][4]
This is different from the design of other oval style chainrings, which have the smaller effective chainring diameter coincide with the cranks being at top and bottom dead centre (TDC and BDC), thus making the crank easier for the rider to turn through BDC for a constant chain tension. By having the chainring at its peak effective diameter with the cranks level, where the rider has maximum leverage over the crank during the power stroke, these designs are supposed to make better use of the rider's power output.[3]*
To be honest I don't really get it.
Had a Cannondale with Biopace back in the day but can't remember what that was like exactly.
Have a SS with a Biopace chainring in the front right now as well because, well, the bike came with it and I couldn't be bothered to swap it for something proper, also my chain is slacking like a fucking hammock so yeah.