ok, i''l try to explain it better. follow actions make it so when the sample ends, u can have another row automatically launch (triggered at your global quantization rate, as if you'd actually triggered it manually). for example, if u set row 2's play mode to 3 and set row 3 to 2, you'll see that when the sample on row 2 ends, row 3 is automatically launched (and vise versa)
spanning samples across multiple rows- for example, assign the same sample to row 2 and 3. use 'snap to grid' (option-drag the waveform or choose grid from the 'snap to' menu) and select the first half of the sample for row 2 and the second half for row 3. again, set row 2's play mode to 3 and set row 3 to 2. trigger row 2 and you should see it working.
hope that helps.