Tap to expand stack, usually of icons (into a Grid is common), etc. See all ways of doing it on Surface ... Many other drilldown methods' Antipatterns: do not use a generic icon/graphic to describe the stack, i,e. iphone grouping, be sure to indicate it's a folder very clearly.
Note that the drilldown is partly just opening the stack, and partly clicking the Link or Icon (image that the stack is composed of).
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