Sensa Speed is PP.
The handle has nothing to do withe pull-back problem. Sometime when you use your mower, grab the crossbar below the sliding handle with one hand and push on the handle with the other. The drive then becomes 'manual' and would be good when going uphill. Now imagine a handle attached to the crossbar that your fingers could reach with your hands on the sliding handle - when you wanted the mower to pull you, you could just squeeze the sliding handle down.
The gearing on PP is much higher than the manual control 'Variable Speed', so it's possible for a PP mower with an assist handle to travel at 10 MPH or more. The reason for the difference in gearing is complicated, but it has to do with the fine balance on PP between the force on the handle and the resistance of the mower. The belt slips all the time because it is not possible to push the sliding handle hard enough to tighten the belt all the way, because the harder you push it, the faster the mower is pushed away from you, like any push mower, so the belt never gets tight. Consequently, the gearing has to be high, with belt slip, to get adequate speed with normal pressure on the handle. When you override all this by squeezing the handle down, the mower can develop tremendous speed. I used to leave two black marks on concrete with my old Lawn-Boy 10550 by grabbing the crossbar and shoving the handle down. The assist handle is a way of easily accessing all that power for when you might need it, like going up a steep hill. I'll tell you this, the mower is capable of outrunning you.

I would have to compare the features on the two mowers you mentioned, but I believe the LB in question has some differences from the Toro. The LB is 20" cut and has rear-wheel drive and 'Easy Turn' front wheels which really do make it easier to steer. I don't know exactly which Toro it is, but it may be a 22" cut. I guess the biggest question is RWD or FWD, and the size of the yard. If it were a larger lawn, I might go with the 22". For a smaller yard, the 20" LB.

1) I know about the bar below the sliding handle...in the bginning the idea was to push forward a little with this bar and then the mower will slide back freely...I had as much success with this as just letting it coast forward a bit after I stopped walking.

2) So this extra handle basically allows you to speed up the mower w/out walking faster. I guess I never found a need for this.

Thanks for the info.

Great explanation sax!