TCB interpolation joins each keyframe using a Hermite-type curve. The slope on either side of a keyframe is changed using the TCB (Tension/Continuity/Bias) values.

To set the slope of a tangent, select TCB and change the values in the TCB fields. Each field in the vector changes a different part of the tangent. The TCB fields are only visible in the FCurves Options pane when a keyframe or keyframes with TCB interpolation are selected.

A. TCB settings in the FCurve Options pane B. TCB curves