IN Iaido demonstrations I have seen cuts made by resting a melon on someones bare belly and making the cut. Obviosly none of the volenteers were cut in half, so the practice of pulling the cut is very common and I'm sure perfected by the Samurai.
Indeed that would suck khafra thats why the person suiciding get to chooses who the kaishaku will be, and this is someone they trust that has the ability to easy there pain with 1 cut, not 2 or 3. Virtuous and Gojuman already gave some examples. This is doing able but takes years of practice. Kenjutsu is a very simple beauty art, but it is not easy to master
I noticed the single stab but had artributed it to Katsumoto being so badly wounded that it was all he could do. Very few ever completed the ritual hence the need for the kaishaku. To actually complete the ritual you had to stoically (i.e. with no sign of emotion or pain) completely disembowel yourself. Either by pre- arranged sign or if your kaishaku saw that you could not continue he would end the ritual before you lost honor. Truly a positon of great trust and a great honor to be asked to do so.