Handcuffs are placed so that the lock is away from the hands, as well as double-locked in order to thwart shims.

It's fairly difficult to use a key while properly cuffed, but not impossible however, if you're caught trying to escape you'll be adding another charge to whatever you're being detained for...

To everyone talking about law enforcement: I think that the use of the key is implied for being arrested, but by 'unlawful restraint' they just mean some random person restraining you (kiddnapping?). Some random person probably wouldn't use proper handcuffing technique.

Unless you have very small wrists, they're quite good at this. I do have small wrists, but it's quite easy to make a handcuff tight enough that I can't wiggle around to it. The lock isn't placed on a particular side, it's up to the person putting the handcuffs on to turn them the right way up. And they do - police officers train in and practice this, because their lives may depend on it.