Having the chest strap open reduces the risk of inducing a spin because it makes the harness more sensitive to roll (either input by the pilot or input by the glider) so you are more easily able to detect the lack of lift from the stalling wing as a spin starts.  The feel is that the from normal turning flight you start to rapidly descend, lose speed (and even feel like you are starting to move backwards) and roll towards the inside wing.  Its the last bit that is amplified.  It feels as if the inside wing can no longer support your weight - which it can't because it is stalling.

Having started a spin a wide chest strap makes it somewhat less likely that the risers will get twisted (which can lead to control lock-up as the brake lines get jammed up with all the rest of the lines).

The downside of a wide chest strap is that it makes you slightly more vulnerable to asymmetrics, and it allows the air to roll you about as one side of the wing is lifted relative to the other.  This makes it less comfortable, but actually helps you tell where the lift or sink is.

Cascades: a series of consecutive collapses each triggered by the collapse before.  Commonly; a wing is spun, it recovers, dives and suffers an asymmetric or frontal, then either cravattes or stalls or spins again.  Alternatively, you suffer a blowout on full bar, the wing falls behind you and because you are going so fast it spins or cravattes or stalls before you fall back under it.  You then have some pendulum motion on top of a collapse of some kind to sort out.  In almost all cases cascades are the result of imposing some large dynamic pendulum motion on top of some smaller instability.  To sort them out you have to damp out the dynamic motion.  Most easily done by stalling the glider!  (Stalling is also likely to untangle any cravattes).  Obviously if you haven't got a few hundred feet (at least) you are best off to look-grab-twist-pull-throw your reserve and remember to let go!

Cheers,
Adrian