This is a new experimental feature in 4.16 that makes it possible to animate a rig by setting keyframes in a sequence on a Control Rig (IK effectors) and also baking the finished animation into normal animation sequences. I'm really excited about this so I made a quick guide. I'm not an expert though so if something's wrong don't hesitate to correct me.

Getting started

1. Enable the plugin in Plugins: search for control, it's called Control Rig.
2. You need a character - SK_Mannequin from the anim starter pack works fine. Put it in the level for now.
3. In the modes panel (place, paint, landscape, foliage etc) there's a new button called Animation. If you click that you'll see a few new options: Sequence and Actor. First click the eyedropper for Actor and choose your character. After that click None in Sequence and create a new ControlRig Sequence.
4. In the sequence that popped up press Add and choose ControlRig - BaseHuman. You should now see a bunch of red, blue and gray shapes appear in the level and a control panel to the left that shows the bones of the character.
5. By manipulating these widgets you can move the character using either IK or FK. By clicking the yellow IK buttons you can switch from IK to FK.
6. You can use auto key to set keyframes when you move the controls. I don't know if there's an easy way to set keyframes using the control picker to the left or selecting a single control in the viewport.
7. When you're done animating you can export your animation to an animation sequence. To do that click the box with an arrow in the sequencer window. Select the skeletal mesh, create a new animation sequence and click Convert and it will get exported.

Using a custom character

If you want to use custom characters you may have to set up bindings for your character. To do that go to the skeletal mesh editor and open Control Rig under Window. Press Add New and select Show Plugin content and Show Engine content in the view options. Then click BaseHuman. Then you'll have to select the correct target bones for each control bone. I guess it would be nice if this could be done automatically if the names match like in the retargeting manager.
There's also a Control Rig component which can be added to blueprints, but I'm not sure when this is needed.
It should be noted that the BaseHuman rig seems to expect the bones to have a certain orientation which can mess things up if you're using a skeleton made in Blender for example. My character turned into a Bloodborne style monster which could be useful if you're making a game like that but in most cases it's not very useful. As a workaround you can create the animations with a character that works perfectly like SK_Mannequin and then retarget the animation sequence when you've exported it.

This thread is for feedback/testing and general discussion. I will post some thoughts after more thorough testing!