By default, yiimp will adjust the difficulty of your miner automatically over time until you have from 5 to 15 submits per minute. It's a good trade off between bandwidth and accuracy.

You can also set a fixed custom difficulty using the password parameter. For example, if you want to set the difficulty to 64, you would use:
-o stratum+tcp://unipoolia.com:3433 -u wallet_adress -p d=64

Here are the accepted values for the custom diff:

Algorithm	Min	Max
Scrypt, Scrypt-N and Neoscrypt	2	65536
X11, X13, X14 and X15	0.002	0.512
Lyra2	0.01	2048

If the difficulty is set higher than one of the a mined coins, it will be forced down to fit the lowest coin's difficulty.