If you have an office/executive chair with a screw jack mechanism and want to adjust the height, the base has to have the weight taken off it. Simply flip the chair forward so the base in free and in the air, then turn the adjusting ring left or right with your hand, it will turn easily. Stand the chair up and you're done. Repeat as needed to get to the height you want.

Click image for larger version. 

Name:	Swivel.jpg 
Views:	405 
Size:	181.7 KB 
ID:	14373