It's a total different way. LASS legato are very "tight", in my opinion, which allows fast playing, but lacks of the transition sound. This tweak is amazing and makes the legatos way better for slower stuff, it's night and day. It took me a lot of time to figure this out.
I'm not at my computer, but I try my best:
1) Open WRENCH mode.
2) Select all the legato interval samples with the SHIFT key (they are called this way, I believe it's 12 ones for up/12 ones for down intervals) and set the edit flag (the way rottoy described it).
3) In the SOURCE area, choose SAMPLER mode instead of DFD. This allows you to change the sample start, but will load the samples into RAM, I believe. I'm not an expert if this is the correct way, sorry to say!
4) Click on MOD. Select CONSTANT as modulator.
5) On the right side, choose SAMPLE START instead of PITCH.
6) Click on INVERT <> so that it's highlighted in yellow. Now you can move the slider to the right. The more to the right, the more of the transition is heard. This means the more to the left the sample start is pushed in the WAVE EDITOR.
OPTIONAL: In step 4 you can choose e.g. VELOCITY instead of CONSTANT as modulator, so that each time you hit a key, you get a different transition, and not the same. The slider defines the maximum value. If you click on MODULATION SHAPER next to INVERT you can define a response curve, too.
Often it doesn't work and I have pitch problems when choosing SAMPLE START in step 5. So it's better to select only interval 2 up to 12 with the SHIFT key first and add interval 1 as the last one to the selected group. Don't ask me why, but it works.