Im going to take this to the extreme to show a slimming technique that I use. I have always used the warp tool in Free Transform. I do not use liquify. Warp is fast efficient and really easy. This tool is for the little nip/tuck the slimming of the arm. The double chin you accidently caught etc and small corrections that make a big difference to the buyers of the image.
1. Lasso the area you want to move.
2. Copy and paste.
3. Ctl T Free tranform
4. Right click Warp
5. Move the edit points to where you want to warp. Accept.
6. Check for edit point or break points, adjust these by going back into warp. OR
7. Go to the erase tool and lightly feather the edge of the layer at 10 %
8. When you are happy flatten image.
9. Use this responsibly DO NOT over slim it’s unrealistic and looks wrong. DO NOT lose the ‘real’ shape of an arm/waist etc.