Aaaaah, I just googled what a slant board was (I’d never heard of one). It looks like this would add a a pretty intense calf stretch to your forward fold, as well as apply a bit of tension to your sciatic nerve - so no wonder you feel more intensity when you stretch using one! (Assuming you’re using it to basically lift/angle your toes UP and not down).

Stretching your calves by elevating the ball of your foot can make a huge difference in a forward fold (I even have a video of how much I’like 30s of calf stretching helped my standing split in a single practice). But you’re right, it kind of feel like it “fires up” the whole back of your leg - it’s very intense!