IK bone constraint: curves or not?

I didn't find anything when looking at the documentation, so maybe someone has experience.

I'm trying to animate a straightforward motion with two IK bone constraints and two IK-handles:

• Hand moves toward an object (moved by IK#1)
• Hand arrives at object. IK#1 influence turned off, IK#2 influence turned on
• Hand and object continue moving together (both moved by IK#2)

My confusion is: the Influence value looks like a float 0.000 ... 1.000. But if I insert keyframes to create curves on the 2 influences (hoping for a smooth transition) there is no curve behavior. Value of 0 acts like 0 and anything greater (like .01) acts like 1. Probably this is normal; I just never encountered it before. Adjusting the bone positions and using a single-frame influence hand-off (no curve) seems to work.