A likely way to create this look is a combination of a very bright white background, a slow shutter, and flash.

The basic setup is:

• Strongly light a white backdrop so it records as pure white.
• Use a slow shutter speed so moving people blur during the exposure.
• Keep ambient/front continuous light relatively low so the blur stays soft and grey rather than fully exposed.
• Add flash, often on rear-curtain sync, to freeze part of the subject at the end of the exposure.

That gives you two layers: motion blur from the long exposure, plus a more defined “burned-in” shape from the flash. If the face or body is more visible, that area was likely hit by flash; if a figure is mostly dark, the flash may have been aimed mainly at the background instead.

Multiple flashes could create multiple contour-like traces. Some images in this style may also involve post-processing or masking/blending, especially if the separation between tones looks unusually clean.