Most legal blocking below the knees occurs by the interior line against the defensive interior line as soon as the ball is snapped. The offensive blockers just fall over the defender's feet.
a. The main reason for this is the restriction against blocking below the waist once the ball has left the free-blocking zone (FBZ). The reality is that the ball leaves this zone VERY quickly and almost instantaneously when in a shotgun formation.
b. The FBZ is 8 yards wide centered on the ball and 3 yards deep on either side of the ball. This may seem like a large zone initially (8x6.3 yards), but:
c. The additional restriction of both the blockee and the blocker being on their respective lines of scrimmage at the snap really cuts the potential "targets" for this kind of block down quite a bit. Unless a linebacker is standing in a gap on the LOS at the snap, he can't be targeted for a low block, even if he was within the FBZ at the snap.
The bottom line is, in almost all cases of a legal block below the knee, no one has finished taking even one full step yet. The risks to the knees are minimal. Backs cannot use this block because they aren't on the LOS at the snap. Ditto for a defensive end trying to strip the lead blockers on a sweep even if he was on the LOS and in the FBZ at the snap. (the ball already left the FBZ in all likelihood). The number of players who can use it, the number of players it can be used against, and the very limited window where the block can be legally used almost DICTATE that the block is delivered immediately at the snap against the guy right across the LOS.