You don't need the tractor running or a hydraulic implement attached. Simply put, a "float" valve is technically a 4 way valve, meaning it has 4 distinct positions.
1. Normal, spring loaded, static position is centered with no hydraulic action.
2. Moving the handle one direction, the handle will move against a spring until it reaches the end of it's stroke where it would cause a cylinder to extend or retract. This position is lost when you remove your hand from the handle at which time it will return to the original static position number 1.
3. Moving the handle the other direction it will come against a spring until it reaches resistance and stops moving where it would cause a cylinder to extend or retract. This position is lost when you remove your hand from the handle at which time it will return to the original static position number 1.
4. Moving the handle in the direction of position number 3 until it reaches resistance, then apply more pressure until the handle "detents" to a further position. This position is retained by the detent even after removing your hand from the handle. This position "floats" the attached cylinder, allowing fluid to freely flow back and forth. This position is used to allow a snow plow to ride over uneven ground or in the case of a loader bucket, you can back drag the bucket, smoothing uneven ground without lifting the front of the tractor or lifting the bucket off the ground because of changes in elevation. To get out of "float" position you simply return the lever to position number 3 and let go. The lever will rest in position number 1.
I'm guessing all marketed loader systems will have the "float" feature controller for the bucket lift circuit.