The Stop-Start technique is basically aimed at achieving a delayed ejaculation and is practiced as a tool to prevent Premature Ejaculation (PE).
It works like this: one stimulates the penis (either through masturbation or sexual intercourse) until the urge to ejaculate arises. At that particular point, just before the-point-of-no-return, one stops the stimulation and waits until the sensation passes (which can be somewhere between 5 and 20 seconds).
After that, one repeats the routine several times, that is, one again stimulates the penis until the urge arises and then abruptly stops.
Mind that, in the case of sexual intercourse, you don’t need to actually pull out your penis (that could spoil the moment for your partner); just stop thrusting or rubbing and, together with your partner, focus on other things (kissing, talking, changing position, or whatever is pleasurable).
The idea is to really practice this. You should train this technique several times a week, over a longer period, in order to get better control over an ejaculatory orgasm during sex.