In a thump the jet positions itself slightly in front of, and under one wing. Works well on a prop aircraft, as the airflow is disrupted across the wing, making the plane fall, rather abruptly, 20 to 60 feet. Once it is clear of the wash, normal flight resumes. However, the jet can reposition itself again, for another thump.
There is little risk to the aggressor in this case.
From an ABS news interview, http://abcnews.go.com/Nightline/story?id=128859&page=1
Q: Isn't there a certain amount of gamesmanship going on at least in
your experience with the Russian pilots?
A: There was. Most times it was a very straight-forward operation.
They would come join on us, see that we were the same airplane that
was out there the day before probably, and then leave. Every once in
awhile they would get a little closer, stay a little longer and on two
occasions they did what we call a thumping maneuver — where they'd
come up underneath us and then pop up in front of our nose and put us
into their wing tip vertices and their jet wash and bounce the
airplane. And one time they bounced so much that the auto pilot kicked
off and we started to make a turn that we hadn't planned on and we had
to quickly react to that.
Q: This thumping occurs when they are flying right underneath your
aircraft and then pull up in front of you?
A: Yes. What they do is they come up right underneath your aircraft
and then pull up right in front so that the wing tip vertices and the
turbulence from their wings and their engines go right in front of the
airplane and get the fuselage and wings of the P3.
From the book, The Chinese Navy: Expanding Capabilities, Evolving Roles
edited by National Defense University (U S ), Institute for National Strategic Studies, Phillip Charles Saunders, Christopher D. Yung, Michael Swaine, Andrew Nien-Dzu Yang
“In a spiraling exacerbation of tension, the Chinese became
increasingly reckless in their responses to American reconnaissance
missions. PLA Navy Air Force (PLANAF) pilots began harassing American
pilots, either as ordered from above or on their own volition, often
employing alarming closure rates during intercepts and sometimes even
pulling ahead of the larger aircraft and “thumping” it with engine
afterburner exhaust. (*26) Finally, on that fateful April Fools' Day
2001, “hot-dogging” Lieutenant Commander Wang Wei captured the world's
attention when he struck a U.S. Navy VQ-1 EP-3E on his botched attempt
at a third “join-up.” … The pilot of the EP-3, young U.S. Navy
Lieutenant Shane Osborn, recovered from a steep 8,000-foot
uncontrolled dive and flew toward Lingshui on Hainan Island in his
badly damaged aircraft (*27) Osborn made an engine-out, no-flap
landing without an airspeed indicator at the PLA Navy airfield...”
And this from Congressional Research Services, Order Code RL30946, China-U.S. Aircraft Collision Incident of April 2001: Assessments and Policy Implications
On several occasions, PLA pilots had overtaken EP-3 aircraft from
the stern at high speed, passing underneath and abruptly pulling up
in front of the American aircraft at close range. The practice known
in some pilot circles as "thumping" is essentially an aerial tweak,
intended to cause consternation to the victim as he is suddenly
confronted with the noise, jet wash, and the jet in unexpectedly close
Normally thumping does not make the media, but in 2001 an incident involving the EP-3 suffering significant airframe damage elevated this to public light. While journalists, media and other non-pilots may comment on thumping and simplify it to be wake-turbulence, or whatever, it should be clear that the thumping maneuver is intended to induce loss-of-control, even if only temporary, on the victim aircraft.
Furthermore, contrary to press reports, the maneuver is more effective when performed asymmetrically on the victim aircraft, as it can induce a unilateral stall-spin due to the loss of lift on one side of the wing, which increases the "effectiveness" of the maneuver.
A former USAF one-star indicates that more than once he had fleet airframe damage and had to remind pilots to avoid doing such maneuvers during training sessions. He also indicates that there is no SOP or written guidance on this procedure, as of his retirement in 2006.
In summary, thumping is not a normal wake turbulence maneuver. It is not a ICAO style intercept maneuver. Executed asymmetrically, this deliberate maneuver has a very significant aerodynamic effect in that it causes loss of lift on one (or more) wing, unexpectedly. While on low approach, it is extremely risky. The maneuver is not documented as a standard maneuver, nor is it officially sanctioned. There are numerous memos discouraging or prohibiting the practice of the procedure. A Vietnam era F-4 pilot recalls the maneuver being used in Vietnam, but does not know when it was first employed. He points out that the capability of the F-4 and other Vietnam-era fighters facilitate the maneuver.