n/b strap? What is it?

[tpoj]

There is an option in Abit IS7 motherboard
to choose N/B strap from PSB400 , PSB533 and PSB800

Can someone explain in technical details, what does it do?

 

[AnImuS]

lets the mobo know which fsb the type of proc is:
400=100x4Quad 533=133x4Quad 800=200x4Quad