What you are asking about is a front leg sweep and I was known for using them very effectively in sport karate especially. What you are trying to do is disrupt your opponent's balance so that you can take advantage of him. He has no punching power basically without his balance and can't do much anything else either until he regains it. I often had to grab my opponents or help hold them up after I swept and scored on them because if they fell then I could be disqualified for using a technique with too much chance of injuring someone and not exercising control with it as well.

You generally can't sweep someone unless they have their weight off their front leg some or are just starting to put it down. That being said I would set it up or use it a couple of different ways. Out of a southpaw stance so that my lead leg was close to theirs I would throw a stiff jab aiming higher and more at their forehead. That would get their head and the weight of it going back on their heels a little and I would not then retract the jab all the way but instead grab the shoulder of their gi and then push while punching with the other hand to either the floating ribs or sometimes the face depending on where their hands and arms might go when their balance was displaced.

With really bigger guys I would sometimes not even throw the jab with the idea of hitting them so much but instead just distracting them and then pushing that shoulder to get their weight back off that lead leg and then sweep and hit them.

At other times I would wait until they did a kick with their lead leg and then as they were setting it down would sweep them before they got their weight back down on that leg. Then I always had to grab the shoulder of their gi and help keep them up while also scoring.

I have taken people down with a sweep before in street situations also so it can be a very effective technique but takes some practice in doing and most don't practice it enough to be good at it. One of the ways you defend against a sweep is to put more weight on your leg. Also as far as fighting goes if your opponent is sweeping you or trying to then that's a good indication that you are either not moving enough or being aggressive enough and increasing either of those things can make it harder for your opponent to sweep you.

Here is the basic sweep combined with a grab. Your variation uses a strike in place of the lead arm grab, so the angle will be different: