Which large cover opens first in Block Out Level 668?

The cyan 4-cover on the right opens before the dark-blue 1-cover on the left.

Why keep the middle seam open?

Both the right reveal's brown numbered L and the later left-chamber pieces must cross that narrow connection.

Block Out level guide

expert 10 colors

Level 668 pairs tall blue covers marked 1 and 4 with numbered brown L pieces; drain the right chamber before opening the left.

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Move the outlined purple square left, shift the framed green square away from center and clear exposed left-gate pieces.
Separate the red and cyan bent pieces, then route reachable yellow and cyan columns through bottom/right gates.
Keep the brown 17 L on the left edge while the lower exits reduce its number and both cover states.
Open the cyan 4-cover first and pull its yellow, green, orange, pink and cyan pieces down without filling the seam.
Move the revealed brown 2 L through the right-side space after the cyan lane is empty.
Continue right-chamber exits until the dark-blue 1-cover retracts and exposes the upper-left wave.
Route blue/cyan right, orange/pink sideways and yellow toward top/bottom while the chambers share one field.
Clear the upper green/orange/pink groups before moving the isolated lower yellow vertical and green L.
Exit the final lower green/yellow pieces and remove the remaining blue, pink and orange blocks.

Colors in this level:

Dark-blue, Cyan, Green, Lime, Red, Yellow, Purple, Pink, Orange, Brown

Parking the long cyan column directly beneath the seam when the right cover opens.
Trying to move the final green L while the bottom-left yellow vertical still blocks its approach.

Board notes 5 details

Layout: Dark-blue and cyan covers fill the upper half. The lower starter contains red/cyan bends, lime/red/yellow columns, purple blocks, a framed green square and a brown textured L marked 17.
Goal: Reduce the starter, open the cyan right chamber, pass its numbered brown piece through the seam and then clear the dark-blue left reveal.
Opening: Move the outlined purple square left, shift the framed green square off center and separate the red and cyan bent pieces.
Danger Zone: A red bend or long cyan column under the cover seam blocks the brown numbered pieces; a late yellow vertical can also trap the lower green L.
Mechanics: Two separate tall covers marked 1 and 4 open around brown textured numbered pieces that must cross their shared seam.

Which large cover opens first in Block Out Level 668?
The cyan 4-cover on the right opens before the dark-blue 1-cover on the left.
Why keep the middle seam open?
Both the right reveal's brown numbered L and the later left-chamber pieces must cross that narrow connection.