It’s an option from her Gale (auto-evade) stance.
For instance, after 6BA, Talim will end in Gale (you’ll notice her swinging her weapon around for a moment after the attack). If you hold a direction during Gale (6, 4, 2 or 8) AND the opponent attacks, she will auto-evade into the appropriate option. For the case you are curious about, it’s her 4 option to Wind Leap. (2_8 leads to BT etc, 6 to wind sault.) If the opponent does NOT attack OR you do not hold a direction, the auto evade will not occur and you will shift back to a nuetral state.
So again, for your case, have your opponent block one of your attacks that end in gale like 6BA. Then hold 4 and have them attack you immediately while you are in Gale. If successful, she should automatically auto evade backwards into Wind Leap. You can then press B during the Wind Leap to initiate the attack. It’s basically just like her normal 66B, only it guardcrushes.
Other attacks that end in Gale are…
There are probably some others, but I don’t play Talim at all. Hope that helps. For more info, read up on some of her threads in the sc.com forum.