Coil bind is when the valve spring is compressed fully to the point that all of the coils are "stacked up" on top of each other.

Coil bind is a catastrophic condition that will result in valve train failure. 

If coil bind is happening then you must disassemble each spring and check all the springs (both inner and outer ) 

If there is not 0.060" - 0.100" minimum of clearance between the coils, the solutions are, installing a (+) valve retainer or using (+) Keepers, Installing longer valves, or the spring must be changed.

Keep in mind that these modifications will change the valve spring installed height.