I wonder if there is a feint to make him overextend at that point. He would be off balance. He may be able to move the sword. He would have a harder time moving his body.
If he's overextending you're too late.
A fleche is a type of running attack best executed at pretty short distance, the touch ideally being delivered about the time (possibly before--sometimes after) the rear foot lands, having 'crossed over' the front. The classic fleche looks like someone way off ballance reaching forward with both feet behind the central mass of the body. (You can probably get pics by goggling it).
Most fleches don't look anything like a classic fleche. Rather, they just sort of explode forward like a runner out of the blocks. Tactically, this style of fleche is meant to take one shot and end the action by passing the opponent as quickly as possible so as not to get hit with a parry-riposte. It's very effective, especially if you don't have particularly strong infighting skills, or you're fencing a rather dangerous opponent and you don't want to be in front of his blade too long after closing.
Shorter fleches will look more like the classic fleche (at least when first launched). The shorter fleche is handy if you're fencing an advanced opponent (who will likely parry the initial portion of your fleche attack) and only if you have strong remise and infighting skills. Often I'll fleche a quality opponent that will pick up my blade in his routine parry-riposte system, but because I can often disengage these parries before he's finished parrying, I can 'slip off' and hit him with one or two remise shots before he can finish his parry and get his point back on target (by this time there may be 15-20 inches between us.)
If I can't slip off, and he's coming in with the riposte, I'll automatically take a counte-parry-riposte in 1, as mentioned earlier. After that it's just intuitive, but the action will be over pretty quick. Someone either gets hit, or is off the strip, or you're corps-a-corps.
Attacking on the back is something you'll see more in foil, though it happens in epee too. It usually goes like this:
Opponent lunges to a deep target such as the chest. His blade is picked up in 6 (that would be a counter parry 6 --- capturing the blade with a clockwise (for right handed) or counterclockwise (for lefty) circular motion of the blade. The opponents foible is now at the crux of your forte and guard, but you don't complete a 6 parry which would end up ushering the opponents blade safely off target to the far right (right handed) or left (left handed). Rather, In one smooth, and hopefully swift, motion the hand is lifted upward along with the blade and you make a forward whipping motion or 'Flick' to the opponents back right over his weapon-hand shoulder preferably while his foible is still trapped on your guard.
In foil, you can do this more easily than epee because the blades are far whippier and you have the 'right-of-way' convention that can make this attack practical without taking you're opponent's blade at all. In epee you'd better have his blade or you'll be hit somewhere in the wind up. Again, this is tough to explain, but when you see it it's all very quick (probably about 1 second from the beginning of the parry to the completed flick.)
Obviously, most of this won't be immediately relevant if you're using Boken.
As has been said when he is almost at the end of his extension he is the most vulnerable. I wonder if there is a feint to make him overextend at that point. He would be off balance. He may be able to move the sword. He would have a harder time moving his body.
