I've also used this technique - I have generally hand-blended multiple exposures in Photoshop, using simple layers and eraser-brushing away the area of the car in each previous frame while stacking - it's relatively easy, just a wee bit time consuming - but can be fun to use for sports, motorsports, and other fun uses. A tripod isn't necessary, but can be helpful if you're not real steady, as you don't want to misalign the frames too much or the backgrounds will be hard to match when blending the subjects - they are ghosted, and it will still show some of the background when you have areas like tire smoke and such...so frames where the background is identical will make things easier. Here's a fun one I did at a stunt show:
You also don't have to 'ghost' the subject if you don't want to - when blending the frames, you can actually bring the subject through as a full exposure, stacking only the areas around the subject...like this 3-shot of a jumping motorcycle that comes off looking like 3 bikes jumping:
To the photographer who took those photos - nice work - it's a neat effect and works well for that subject of rally, as you get to see the maneuvering of the vehicles and control of those drivers.
By the way, there actually ARE some DSLRs that can perform an effect like this straight from the camera, though it wasn't the intention of the mode. Some Sony and Pentax DSLRs have an HDR mode, or a multiframe ISO mode, designed to stack frames in camera for dynamic range, or stack frames in camera for noise reduction. Those modes are typically used with stationary subjects, but if used with a moving subject, it will capture the ghost of the moving subject in the previous frames. Sony and Pentax both have a 3-frame capture for HDR, and Sony has a 6-frame capture for noise reduction - so you could have as many as 6 stacked frames with ghost trail captured, during the short 1 second burst.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 200-500mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony NEX5N / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / Pentax K adapter / Konica K/AR adapter / bunches o' Konica & Pentax lenses!