WARNING - What you are about to read DOES NOT take the place of qualified training!!
Whether you're on line or in low or zero visibility, hand squeezes are another excellent way of communicating with your dive buddy. When the conditions are "pucker-factor twenty" and counting, it's nice to know your partner is there and only a squeeze away. It might sound funny, but if you've been there, you know what I mean. If you haven't, take my word for it.
DIVER TO DIVER
One squeeze - Stop
Two squeezes - OK
Three squeezes - Special
Four squeezes - I have found it
Five or more squeezes - Emergency
Keep in mind that you might have a need for special signals like "go get a lift bag, bring down the body bag." In hand squeeze signals I've reserved three squeezes for that. Make sure that all parties involved understand if special signals are being used and what the signals mean.
You can use squeezes in zero visibility as well off line by squeezing your buddy's wrist twice, asking him if he is OK. He in turn would grab your wrist or arm and squeeze twice if he was OK. Another example for going up (ascending) in zero visibility you would grab his thumb and pulling up twice, which asks if he is OK to go up. For UCI divers who really do this stuff, in real normal conditions, this type of communication capability is a must. So practice!