I think that the possibility of a pure upwards release is very design or brand/dependent. With my Shimano SPD-SL pedals, you can in theory pull out of them vertically, although the design is such that the force to release sideways is designed to be very much lower than what it would take to release straight up. In principle, the "clamping" part that holds the cleat in place has a sloped top surface, so that you get some leverage against the spring force when you step down to click in. When releasing sideways (intentionally), the angled shape of the back corners of the cleat gives you a sort of ramp effect, providing leverage to overcome the spring tension of the pedal. The underside of the clamping part has a square cut, however, so that in principle an upwards movement of the cleat is held solidly. But if the cleat flexes a bit, or if the back edge of it is worn a bit, you will get a slight slope of the cleat surface which could result in release, given a sufficiently-high upward force (and perhaps an upwards force that is beyond any normal pedaling motion). The wear-level of the cleat is the biggest variable, as is the spring tension setting of the pedal itself. My belief is that any Look-style pedal, as well as SPD's and SPD-R's, will also have some possibility of upwards release, at a force level that is designed to be well above the normal sideways release force, but can degrade somewhat due to cleat wear, etc.Other pedal types will vary, due to the geometry and design of the "clamping" mechanism, as well as the cleat shape and material. It's a lot like ski bindings. Some designs are such that they respond to forces in all directions and axes, whereas some designs release more easily in some directions than in others. There are also differences in the "elasticity" of the release - basically, how far out can you partially release, and still come back to the retained position, if there is a transient force that goes away? Some bindings do a good job of absorbing short-term transient forces by partially releasing and then coming back, whereas with other designs a sharp jolt will just pop you out of the binding. Bike pedals are more or less overly-simple versions of partial ski binding. (Sorry about going on about this - I just am interested in mechanical designs!)
