Len Rig & Speed Rig. Both rigs use two methods of key morphings.
This method is used in most of our rigs. You need to create 3 necessary keys, which start from squash state in the first key to stretch in the third. The following two keys would be alternative squash and stretch states.
First 3 necessary keys and the last 2 optional
1 key - Squash State.
2 key - It could be an original shape without distortions or like a middle state.
If the shape is leg or hand, it would ideally make it relaxed, without squash and stretch.
3 key - Stretch State
Every following 2 keys will be alternative squash and stretch states.
4 key - Alternative Squash State
5 key - Alternative Stretch State
You can find Rig Mode (1-2) option in every rigged shape layer in the Length Rig effect. Value of 2 will change rig to the second type, where only first and third keys will be used in the rig, without the middle state. So 2 key will be ignored.
1 key - Squash State
3 key - Stretch State
Every following 2 keys will be alternative squash and stretch states.