The Mark and Register tools on the DopeSheet allow you to align (register) poses and animations.

Note: There is a graphical Paste Register example here: PasteRegister example.

Frequently, when animating, poses must be lined up so the motion looks correct. Sometimes the poses were created in different positions, or maybe you simply want to move an entire animation sequence to line up at a particular point. Example: when building a walk cycle, you will want the planted foot to always be in the same position to prevent sliding.

The names mark and register come from traditional cel animation, where registration points are used to line up cels.

Select a single joint and click Mark. Now that joint is marked at that position. If you select a different frame and click Register, then the entire skeleton is moved in the new frame so that the joint lines up with it's marked position.

If you select a range of frames and click Register, the first frame is registered just as above, but then the rest of the frames are moved the same amount, keeping the animation intact.

Example:

Say a walk cycle has four frames, and the cycle begins and ends with the left-foot planted. Go to the Dope Sheet and select the last frame of the cycle. Select a joint in the left-foot and click Mark.

Now copy and paste the needed frames (if you play it back now, the character slides back to the start point at the transition between the cycles). Select
the pasted frames and click Register. The joint (left-fott) that was marked is used as a registration point for the first pasted frame, and all the other frames are adjusted to keep the walk cycle intact.

PasteRegister

The paste register command is a shortcut for quickly extending an animation cycle. Click PasteRegister, and (1) the most recently copied frames are pasted into the selected frame, and (2) the pasted frames are registered using the most recently marked joint.

Example:

A walk cycle has been created. The walk cycle ends with the right-toe touching the ground, and begins with the right-foot planted. Therefore, the right-toe can be used as a registration point. The right-toe joint is selected, and the Mark button is pressed. Then, the walk cycle frames are selected in the DopeSheet, and the Copy button is pressed. Now, a target frame is selected, and PasteRegister is clicked. The walk cycle frames are pasted, starting at the selected (target) frame, and the frames are automatically registered using the marked right-toe.

Note: There is a graphical Paste Register example here: PasteRegister example.