I'll use Renkai as an example.

1. Press down for a regular hit. (1 hit)
2. Press right (with proper timing) to Just Attack the first combo of Renkai (3 hits)
3. Press right again (with proper timing) to Just Attack the second combo (2 hits)
4. Press right again (with proper timing) to Just Attack the third combo (4 hits >.> I think)

The only criticals you get are following correctly timed button presses. Oh, and you still can get criticals like before if you choose not to JA, but that's not so important.