track backward using shifting

- track backward and stop
- drag move (select mode) the tracker box (dotted) to new pattern
- switch to 'add' , drag the ref box to new pattern
- click 'snap'  , the cross icon will jump to ref box position, and ref box will jump to cross original pos (switch position)
- switch to 'select' and move the ref box back into the new pattern
- go into anim curve, select the current keyframe of ref channel, copy and paste it onto the 1st frame.
- go back to last position, click 'step' once. Fix the position of the track box so it will lineup to the correct direction.
- analyze

if i want to shift again :
- stop
- click 'snap' , switch to 'select' move the boxes and cross together to new pattern.
- open anim curve again. Delete the keyframe of ref channel in the 1st frame. Copy from previous frame , the new ref channel keyframes. Paste it into the 1st frame.
- click 'step' and fix position of the track box again.
- analyze

if i want to shift more , repeat the second block of instructions above.

IMPORTANT NOTE :
Couple things that may drag into problem :
1. Delete reference keyframe in the reference box only delete the X and Y, but dx dan dy has to be deleted also using channel editor.
2. Remember in some case, we need to set keyframe at frame 1 to hold the whole keyframe.
3. When tracking backward, after each 'SNAP' , we have to copy keyframe (ref) and delete the first key and paste the copy. DO NOT REPLACE - but - DELETE FIRST & PASTE.