I love it when my husband ties me up, blindfolds me, whips me, or just uses "force" (in a playful way of course). We have an understanding though - if I say stop - he stops. Instantly. I enjoy a little pain with my pleasure, but he's careful not to go to far. In fact I'm typically asking him to be more rough.
There is a good product out there. Bed straps. They hide nicely under the mattress and have velcro straps. I enjoy my hands tied, my feet not so much, but I let him do it every once in a while.
Handcuffs do hurt if you put them on too tight, I like mine a little snug - but beware they DO leave marks if she moves too much in them.
A good idea is to have a code word that REALLY means stop. Sometimes I'll say no, but only because it's so intense, not because it's unpleasurable. So for us, even thought stop generally means stop, if he asks if it's too much and I say "its yellow" it means it's getting to be too much but its not there yet. If I say "it's red" that means back off, slow down, stop, etc.
If it's green that means I can tolerate a higher level of intensity.