During a conversation on the phone, it is the act of simulating a "High 5" by pressing '5' on the keypad at the appropriate moment.
Joe: So where did you go with that girl last night?
Moe: Back to my place.
Joe: Brown chicken, brown cow?
Moe: You know it!
Joe: PHONE 5!!!
*Joe presses 5 key*
A word blending the sounds and combining the meanings of two others. Formally known as a 'Portmanteau'
Combine 'yell' and 'cellular' and you get 'yelluar', or 'Spanish' and 'English' and you get 'Spanglish'. Ironically 'Twerge' is made from 'two wordsmerge'.