The 1-2-2-1 Pattern in Minesweeper

The 1-2-2-1 looks like the 1-2-1's cousin, but its mines go somewhere else. Read it the same way, from each end, and it solves six cells at once.

Step 1 of 4

1 2 2 1
Row 4 reads 1, 2, 2, 1 in columns 3 to 6, under a straight edge of six hidden cells. Read it from the left 1: the first 2 puts a mine past itself, above the second 2. Read it from the right 1: the second 2 puts a mine above the first 2. In a 1-2-2-1, the mines are above the two 2s, starred here, and every other cell above the pattern is safe.
Flag and open
Two mines side by side, with a safe cell on each side of them.
Your move
Flag the two mines, then open the cells above the two 1s.
A 1-2-2-1 in disguise
Row 4 reads 2, 2, 2, 1 in columns 3 to 6. Take the flags into account: the first 2 touches the flag in row 5, column 2, so it needs only one more mine. Read it as a 1.
Your move
Find the 1-2-2-1: flag its two mines and open the cells above its 1s.
Why it works
Which explanation of the 1-2-2-1 is right?
Question

What you learned

Along a straight edge, 1 2 2 1 means mines above the two 2s, and safe cells above the 1s and past both ends.
It is two 1-2s, one read from each end. So is the 1-2-1.
Take flags off a number first: a 2 next to a flag counts as a 1.