It is a bit confusing that some posters are saying that overhang or offset angles "cause" the skating force. The WallyTools videos posted here that shows that, under specific conditions, skating forces can even be in the opposite direction, add to the confusion because they do not attempt to explain why this is so. Overhang and offset angles don't directly "cause" skating forces, but, they do create the conditions that give rise to skating forces because they create the geometry that causes the force of friction (drag) to pull the arm in one direction or another.
The drag developed at the point of contact of the stylus with the wall of the groove is pulling along a line that is a tangent to the points of contact of the stylus. Whether this tangent is in the same direction as the cantilever or to the left or right of this line is dependent on the alignment of the cartridge at that particular moment. For a reference point, imagine that a spherical stylus, when viewed from directly above, is a clock face with the axis of the cantilever bisects the 6 o'clock and 12 o'clock position. When that stylus is at a point on the record where the contact is at the 9 and 3 o'clock position, the drag is directly along the line of the cantilever. Will this result in skating force? The answer is: it depends. If the alignment is such that the cartridge is perfectly straight (no offset angle to the headshell and cartridge), at that perfect tangency point on the record, there is no skating force because that drag is pulling straight back to the pivot point of the tonearm and that pivot is resisting that drag. But, if the alignment is a conventional alignment, such as Lofgren B, at the point of perfect tangency, there is a substantial skating force, because the drag along the same line as the cantilever is pulling at a point to the right of the pivot point (because of the offset angle) and the pivot cannot completely resist that pull. On the setup with a cartridge facing straight forward with no overhang, at points outside of the perfect tangency point on the record, the points of contact will not be at the 9 and 3 o'clock position, but something toward the 10 and 4 o'clock position; this means that the drag direct (tangent to these points) is aimed to the right of the pivot and so there is skating force. At the point inside the point of tangency, the points of contact are toward the 8 and 2 o'clock position, so the direction of drag is to the left of the pivot an the skating force is now in the opposite direction. The reason there is ALWAYS a skating force in the conventional direction with conventional alignments (e.g., Lofgren) is because these alignments minimize the deviation from perfect 9 and 3 o'clock contact (always less than 2 degree deviation) which is MUCH less than the fixed offset angle; this means that the drag is ALWAYs to the right of the arm pivot point even if does deviate over the diameter of the record.
The drag developed at the point of contact of the stylus with the wall of the groove is pulling along a line that is a tangent to the points of contact of the stylus. Whether this tangent is in the same direction as the cantilever or to the left or right of this line is dependent on the alignment of the cartridge at that particular moment. For a reference point, imagine that a spherical stylus, when viewed from directly above, is a clock face with the axis of the cantilever bisects the 6 o'clock and 12 o'clock position. When that stylus is at a point on the record where the contact is at the 9 and 3 o'clock position, the drag is directly along the line of the cantilever. Will this result in skating force? The answer is: it depends. If the alignment is such that the cartridge is perfectly straight (no offset angle to the headshell and cartridge), at that perfect tangency point on the record, there is no skating force because that drag is pulling straight back to the pivot point of the tonearm and that pivot is resisting that drag. But, if the alignment is a conventional alignment, such as Lofgren B, at the point of perfect tangency, there is a substantial skating force, because the drag along the same line as the cantilever is pulling at a point to the right of the pivot point (because of the offset angle) and the pivot cannot completely resist that pull. On the setup with a cartridge facing straight forward with no overhang, at points outside of the perfect tangency point on the record, the points of contact will not be at the 9 and 3 o'clock position, but something toward the 10 and 4 o'clock position; this means that the drag direct (tangent to these points) is aimed to the right of the pivot and so there is skating force. At the point inside the point of tangency, the points of contact are toward the 8 and 2 o'clock position, so the direction of drag is to the left of the pivot an the skating force is now in the opposite direction. The reason there is ALWAYS a skating force in the conventional direction with conventional alignments (e.g., Lofgren) is because these alignments minimize the deviation from perfect 9 and 3 o'clock contact (always less than 2 degree deviation) which is MUCH less than the fixed offset angle; this means that the drag is ALWAYs to the right of the arm pivot point even if does deviate over the diameter of the record.