Sometimes it is not visible and the diagnosis is done by actually hearing the slap. Treated systematically, possible lesion sites causing foot drop include (going from peripheral to central):

Neuromuscular disease;
Peroneal nerve (common, i.e., frequent) —chemical, mechanical, disease;
Sciatic nerve—direct trauma, iatrogenic;
Lumbosacral plexus;
L5 nerve root (common, especially in association with pain in back radiating down leg);
Cauda equina syndrome, which is caused by impingement of the nerve roots within the spinal canal distal to the end of the spinal cord;
Spinal cord (rarely causes isolated foot drop) —poliomyelitis, tumor;
Brain (uncommon, but often overlooked) —stroke, TIA, tumor;
Genetic (as in Charcot-Marie-Tooth Disease and hereditary neuropathy with liability to pressure palsies);
Nonorganic causes, e.g. as part of a functional neurological symptom disorder.If the L5 nerve root is involved, the most common cause is a herniated disc. Other causes of foot drop are diabetes (due to generalized peripheral neuropathy), trauma, motor neuron disease (MND), adverse reaction to a drug or alcohol, and multiple sclerosis. Gait cycle
Drop foot and foot drop are interchangeable terms that describe an abnormal neuromuscular disorder that affects the patients ability to raise their foot at the ankle. Drop foot is further characterized by an inability to point the toes toward the body (dorsiflexion) or move the foot at the ankle inward or outward. Therefore, the normal gait cycle is affected by the drop foot syndrome. The normal gait cycle is as follows:

Swing phase (SW): The period of time when the foot is not in contact with the ground.
FES is applied to lower extremities for improving functional walking in stroke patients;  for the correction of foot drop. They have benefited patients by improving gait speed, muscle strength and other functions.Treatment for some can be as easy as an underside "L" shaped foot-up ankle support (ankle-foot orthoses). Another method uses a cuff placed around the patients ankle, and a topside spring and hook installed under the shoelaces. The hook connects to the ankle cuff and lifts the shoe up when the patient walks. Both these techniques show significant improvement on usage. Identification of a suspect in a murder case
In December 2021, police in Toronto, Canada said they were looking for a person with a distinctive gait as a suspect in the murder of Barry Sherman and Honey Sherman. Medical professionals  quoted by the Toronto Star said that the distinctive gait could  be caused by foot drop. See also
Yoga foot drop
Toe walking
Polymyositis
inclusion body myositis

References
Further reading
Agency for Healthcare Research and Quality (2011). Healthcare Cost and Utilization Project. Balali-Mood, Mahdi (January 2008). "Neurotoxic disorders of organophosphorus compounds and their managements". Arch Iran Med. 11(1):65–89. PMID 18154426. Jokanovic, Milan, Melita Kosanovic, Dejan Brkic, and Predrag Vukomanovic (2011). "Organophosphate Induced Delayed Polyneuropathy in Man: An Overview". Clinical Neurology and Neurosurgery 113.1: 7–10. PMID 20880629. doi:10.1016/j.clineuro.2010.08.015. Mayo Clinic. "Foot Drop". Pritchett, James W., MD (June 21, 2018). "Foot Drop". Vinod K Panchbhavi, MD, FACS (ed.). Saladin, Kenneth (2015). Anatomy & Physiology: A Unity of Form & Function. 7th ed. New York: McGraw-Hill Education. Print.