The HumanIK Controls sliders in the Controls tab let you adjust the amount of influence each solution (IK or FK) has on a particular body part. This gives you complete control over which solution your skeleton follows between the synchronized keyframes.
To adjust the influence each solution has on your skeleton
The cells of the Character representation update to give you visual feedback on IK Blend settings. (See IK cell status.)
For example, a value of 30% is assigned to IK Blend T in the image below. When the character's wrist effector is translated, the skeleton reaches towards the FK rig because it is following the FK solution more closely than the IK solution.