Remember the basic rule of the game:
Same-colour pieces can only touch at their CORNERS.
That makes corners all-important places on the board.
When two pieces of different colours meet "corner-to-corner", it kills that corner for BOTH of them.
That is especially bad in C2 when the two colours are supposed to be cooperating!
First we'll look at a commonly seen case in isolation, then return to our in progress game, for a slightly more complex situation. We leave out yellow and green to focus on the problem.
Perhaps having heard "Blue and Red should meet", this player has just played the Blue W continuing Blue's drive toward Red.
They meet, all right, but not in the right way!
Blue's leading corner hits Red's leading corner, and it becomes difficult for either of them advance!
The problem here is that neither of them advanced far
enough to have a proper side-to-side meeting.
If you are tempted to make a move like this,
STOP! and apply some DDT -- "Don't Do That"!
Now let's return to the game we're following and see how that player "accomplished" the same thing, but in a slightly different way.
Take a look at the Blue W here (ignoring Blue's previous bad moves). At first it may look great, but the problem is that while it's fine for Blue, it's bad for Red, and Blue and Red are on the same team!
This is an easy mistake to make when not being careful: one of our colours blocks the corner of our other colour; we need to be actively avoiding moves like this, especially on turns 4 and 5.
In this case the move is even more deceptive, because it seems like Blue and Red are doing something good together on the right half of the connection.
(Connection refers to colours on the same team meeting, but in a good way.)
The key is to avoid corner-to-corner moves, even when the corner-to-corner isn't the end of the piece. For example, take a look at this picture:
Here we see the essential badness of the Blue W - the corner-to-corner aspect. The fact that the Blue W continues, and isn't just corner-to-corner does make the situation better, but it's still bad for Red.
Blue can rotate the W to end up with a much more favourable move - a full connection with no self-blocks:
Whenever two colours touch side-to-side like this so that both colours can pass each other on each side, we call this a ``full connection'', or just a connection. Connections are really strong, and we should make them (properly) when we have the chance.