On top Guided pistons, the small end of the connecting rods fit very snugly into the slot on the underside of the piston where the wrist pin goes through. There are some thick spacers that hold the connecting rod in place and minimize side to side play. The big end of the connecting rod that attaches to the crankshaft has virtually no side to side play. The side of the connecting rod never touches (or should never touch) the crankshaft....thus it's referred to "Top Guided".
Botton guided pistons have a much larger slot on the underside of the piston where the wrist pin goes through. There is some side to side play. The side to side play is determined by how much the connecting rod moves side to side on the crankshaft journal end. These are referred to "Bottom Guided". Most of the Force motors I've come across are Bottom Guided. Some of the Mercury motors used Top Guided pistons.
Each type of piston requires the matching connecting rod in order to function as designed.
I've never seen an official reason for the two designs, however, I've heard mention that the top guided pistons have a slightly more robust connecting rod and produce less drag. This is probably more desirable in a racing or high output motor, and probably makes no difference in a normal production engine.
