parkourer wrote:<snip>However, if you apply too much tension can't you break the lock? Why would you want to apply so much tension that a twist-flex would bend?
As BillDeserthills pointed out, usually only locks like the Smartkey which has a lot of very weak plastic or pot-metal components in load bearing roles are remotely likely to break under heavy torsion, and even then you are more likely to bend or break your tool before you break the lock. Heavy tension is more likely to screw up your picking attempt. Twist handles will usually bow or bend over time just through normal use.
Heavy tension is required to overcome the resistance from the spring mechanisms in some padlocks, damaged or dirty locks with debris or hardened lubricant blocking plug rotation, or simply because certain locks actually respond well to it. There are also certain picking techniques which require heavy tension at certain points in the picking process.