Maybe I'm misunderstanding, or Wern is.
Frau is talking about a peninsula cabinet with countertop that extends out at a ninety degree angle from wall mounted cabinetry, are you not?
If so, you would place the jack in the bottom of the cabinet, set a 2x4 cut to the right length between the jack and the underside of the counter. Slowly apply enough hydraulic pressure to raise the sag, then insert your support material in the gap. Remove jack and add it to your growing collection of tools.
You could, theoretically, do the same thing with just the 2x4 and a hammer. Wedging the board in between the cabinet floor and the countertop at the approximate center of the sag and using just the board as the support to raise the sag. It would be out of sight inside the back of the cabinet. If it's accessible. Lifting the sag should lower the end with the big gap shown in your photo.
Mickey will be proud. He might even hug you if Minnie isn't around ha ha.
