This can apply to any
fencing weapon
in theory, but it almost always refers to
foil.
A foil is permitted a slight up or down (not both) bend in the blade--no more than 2 centimeters, located near the middle of the blade. This is so that the point of the weapon goes in more
perpendicularly when it strikes one's opponent, whose shoulders because of the normal
on guard stance are rotated away from parallel. It's also to give the weapon some "give," so that it's less likely to snap and break on contact.
After extended use, and particularly with more
flexible weapons, the weapon may start to develop an S curve--bending both up AND down, resembling the letter "
S". This has very distinct disadvantages. It makes the foil's effective length slightly shorter, for one, and it also makes the foil much more likely to break because of buckling
pressures.
It's also
hard as hell to work S curves out of the blade, and sometimes it's more worthwhile (and safer) just to get a new blade altogether. The best way to get them out, from my own experiences, is to take any tool that has an O ring on the tail end (doesn't really matter what the tool itself is, cuz you'll only be using the O ring) and just put the blade inside the ring and start working the foil back into it's normal position. It helps to start bending it below (nearer the
bell guard) where you THINK the bend starts, because
the real problem is always further down than it looks.