Right, the kraeusen doesn't collapse as quickly as with other yeasts, so as the CO2 is produced, it just tends to keep pushing the kraeusen upwards. It seems sticky, like it has more internal structure. If it were bread, I'd call it gluten.
It's my favorite yeast for all Trappist styles. It can perform differently under different temperature conditions. Westmalle ferments it cooler, Westvleteren ferments it warmer. Both work, but give you different profiles. My preference is like most Belgian yeasts; start it cool and let it rise on its own. If you try to constrain the temperature, the yeast tends to be unhappy.