The modern understanding of racecar aerodynamics holds that copious amounts of downforce can be produced by accelerating the airflow under the car, in effect turning the region between the underbody and ground plane into a mobile nozzle.
The Lotus 78 of 1977 famously introduced venturi profiles beneath the car, and sliding skirts to seal the low pressure area thereby created. However, it is less well-known that underbody skirts had fitfully appeared on various cars earlier in the decade. Moreover, it is slightly disconcerting to hear the explanations proffered by several F1 designers from the middle 1970s for the function of these devices.
Gordon Murray introduced inch-deep skirts on the underside of the 1975
Brabham BT44 in conjunction with an overall 'upturned saucer' design, and explains his thinking as follows:
"With any moving form you have a stagnation point where air meets it
and decides how much is going to flow over, below or around it...I
decided, instead of presenting some sort of parabolic-shaped bluff
body to the air, I wouldn't give the air a chance." He sketches a
triangular shape. "That way the stagnation point was there," he says,
pointing to the leading edge of the triangle's base, which is very low
to the ground. "So all the air had to go over the top and you had the
minimum coming under the car," (F1 Magazine, May 2001, p140-141).
Gordon Coppuck, however, had already experimented with skirts on the McLaren M23:
"In 1974 at Dijon-Prenois, vertical plastic skirts around the under-periphery of the car were tried, but they quickly wore away on contact with the track. The idea was to exclude air from underneath the car and so minimise lift," (p49, McLaren M23, Ian Wagstaff, Haynes 2013). The skirts were fitted again to the M23 at some races in early 1976, this time provoking complaints from competitors such as Colin Chapman (!) and Ken Tyrrell.
Talk of minimising lift by forcing air over the top of the car seems misguided because the upper surface of a racecar is generally convex, and the air will tend to be accelerated by a convex surface, producing low pressure on the upper surfaces, somewhat counter to the overall objective.
Nevertheless, it seems that there actually was a beneficial effect to be had from partially excluding air from the underbody, and this is clearly explained by Ian Bamsey in his fantastic book The Anatomy and Development of the Sports Prototype Racing Car (Haynes, 1991):
"The [Shadow] DN8 had conventional wings and a flat bottom and, following the fashion of 1976, it was fitted with skirts along the side of its monocoque, these joined in a vee under the nose. Under certain conditions the skirts rubbed on the track and their general effect was to sweep the air aside, in snowplough fashion. Thus, the overall effect was not one of spatial acceleration of the underbody air, it was one of exclusion. The flow blockage allowed the forward migration of the naturally low pressure air at the back of the car into the skirt's exclusion zone. This was the principle of the so-called open tailed box. A box with the road forming its bottom and only its tail open will experience a pressure reduction within as it progresses along the track," (p59).
So, although the effect may be quite weak, it is possible to generate downforce by excluding air from the underbody.