Since climbing regularly again (!!!), I've observed lots of belaying in the gym. I can't walk up to a stranger and say "Excuse me, sir, I noticed that your poor belaying is totally crippling your climber's ability to try hard, and actively eliminating any hope you had of improvement in this sport." In fact, I don't even want to. When I see bad belaying, it doesn't bother me, unless it's actively courting grave injury. It does make me feel sorry for the climbers, though, because it's really hard to improve at climbing with a bad belayer.
There's a number of issues surrounding poor belaying, but they all tie back to either trust or competence.
The two can operate independent of each other. You can have a competent belayer that you don't trust, or you can trust an incompetent belayer (whoops). Or, of course, you can not trust an incompetent belayer (good idea) or, finally, trust a competent belayer. (Nirvana!).
While you can easily spot signs of an incompetent belayer, it's much harder to determine if they are competent. Then, even if you've established their competence, it takes time to build trust in them.
Here's why bad belaying keeps you from improving - even if you don't actively consider the skill of your belayer, your subconscious knows if they are competent and trustworthy. And if you've not actively considered their skill, you probably don't trust them, and they may be incompetent.
Here are some indicators of unskilled (and untrustworthy) belaying:
- Lets the rope hang between the climber's legs when climber is lower than the third bolt.
- Short ropes climber and doesn't know it.
- Spikes the climber and doesn't know it.
- Keeps rope too tight or too loose.
Now - all of these are easily fixed. So - this is where trust comes in. If your belayer is making these mistakes, and is either unaware or unwilling to fix them, why would you trust them? None of these mistakes puts the climber in grave danger, but all can make climbing much less fun.
If your belayer is competent, and makes none of the above errors, that's enough to make you think they're not only competent, but trustworthy as well. Without trusting your belayer, you cannot push yourself hard enough to improve at climbing.
There's a whole other list related to trustworthy belaying.