Even riskier than a forward bend is the windmill (reaching a hand towards the opposite foot with the other arm straight in the air).
These take the risky unsupported spinal flexion and add a twist, which further compromises the spine. There is really no safe way to do these, and I am not sure what the purpose is. If you are trying to warm up there are much better ways (see post from Sept. 15), they are ineffective if your goal is to work your waist (try a side plank instead; post from Nov. 20). Since they are virtually useless and there is no safe modification omit windmills from your workout.