A square with three options, the pivot, sees two squares with two options each, and all three share one number. One of the three is that number, so a square that sees all three can't have it.
123456789
123456789
2
5
6
4
9
8
5
6
4
3
4
6
8
9
5
2
1
3
67
5
3
9
27
5
267
7
4
5
8
9
4
6
8
1
5
7
1
9
4
5
7
8
6
5
8
7
1
6
4
The pivot and its wingsLoses a pencil mark
A Y-Wing plus one
A Y-Wing's pivot has two options. Here it has three, the shared number among them, so a square must see the pivot too before it can be cleared.
How to spot one
Find a square with three options. Look among the squares it sees for two with two options each, both drawn from its three and sharing one number.
What it gives you
Usually one removal, in the pivot's box or line, because only there can a square see all three.
Practise XYZ-Wing
Boards that need it, with the owl's hints when you're stuck.