Under-layer-only knee pads are designed to be worn directly against your skin or a thin base layer, underneath your ski pants. They're built with a low-profile, sleek fit so they don't look bulky or draw attention on the slopes.
This style is the go-to choice for recreational and all-mountain skiers who want protection without it being visible. Because the pads sit against your body, they stay warm and won't catch on snow or shift around as easily. They also give you a clean, streamlined look that most casual skiers prefer.
The main thing to watch out for is fit—thicker pads may feel tight or uncomfortable under slim-fitting ski pants, so check that your pants have enough room to accommodate them. Also, since they're under your layers, they're harder to adjust or remove on the fly compared to over-layer pads. If you want quick on/off convenience between runs, this style may not be the best pick.