Just finished reflowing a PS3 and figured I would record the nudge test. Some people are a little confused on how to go about it. This is how I do it. I use the TX xclamp removal tool and slide it across the motherboard until I hit the chip so I know the tool is sliding flat along the board. Whence I get to the BGA I just move it maybe 1mm. If it doesn't move easily then it's not fully liquid and do not force it.
Hope this can help someone.
[youtube]http://www.youtube.com/watch?v=KRILHp1McOw[/youtube]