Every chain is a strand of covalently bonded beads (the backbone bond never breaks by heat alone). The crosslink density slider adds extra covalent bridges between different chains. At 0% the chains are just tangled — free to slide past each other. Above 0% they're stitched into a single 3D network, like sulfur bridges from vulcanizing rubber or amine bridges in cured epoxy.
Toggling Heat raises thermal agitation. With no crosslinks, heat lets chains slip apart and the whole tangle slumps into a puddle — a thermoplastic melting and ready to be reshaped or recycled. With a crosslinked network, the bridges hold the chains' relative positions, so heat alone can't make it flow; push the temperature far enough instead and bonds start snapping randomly (charring/decomposing), which is why a thermoset can't be re-melted.
The stretch test pulls the network apart from both ends, then releases it. An uncrosslinked tangle has nothing pulling it back, so it stays stretched (permanent, plastic deformation). A crosslinked network's bridges act like springs anchoring every bead to its original neighbourhood, so it snaps back elastically — the more crosslinks, the closer to full recovery.