How To: Easily Do a 'Slow In' to a Pose

How To: Easily Do a 'Slow In' on an action

Click a frame in-between two major poses, middle-mouse+click-drag it backwards in the timeline, then set the frame.

Congrats, you now have a slow in to the next frame's pose.

This example explained:

Existing keyframes on frame 1, 15, and 40.

Select an empty frame [i.e. frame 33], then middle-mouse+click-drag it backwards [i.e. frame 26] and then set it. Giving a slow-in to the pose on frame 40.